Chapter 4: Kingdom Animalia
In the late 1790s, a strange dried skin arrived in England from Australia. It had the soft fur of a small mammal, the flat bill of a duck, webbed feet, and a broad, paddle-like tail. The British naturalist George Shaw, who examined it, was so suspicious that he searched the specimen for stitch marks. He was convinced some clever taxidermist had sewn a duck’s beak onto the body of a furry animal and sold it as a joke.
The animal was real. We now call it Ornithorhynchus, the duck-billed platypus. But its arrival created a genuine problem for biologists. Where do you place an animal that has fur like a rat, a bill like a bird, and — as naturalists confirmed almost a hundred years later — actually lays eggs?
The answer could not come from appearance alone. It had to come from a set of agreed criteria: how the body is organised, whether it has a body cavity, how many germ layers form the embryo, how the young are nourished. Judged by those criteria, the platypus is a mammal, because it feeds its young on milk from mammary glands — egg-laying and all.
This is what animal classification really is. Not a list of names to memorise, but a set of tests that tell us how living things are built and how they are related.
The variety this chapter covers is enormous, from a sponge that has no organs at all to a whale, as suggested in Fig. 4.0.
Diagram
Fig. 4.0 – Opening illustration: a sweep of animal diversity arranged left to right — sponge, jellyfish, tapeworm, earthworm, cockroach, snail, sea star, Balanoglossus, fish, frog, lizard, bird, platypus — with a small magnifying-glass motif over the platypus.
As you work through this chapter, you will meet the criteria and the phyla that answer every one of these questions.
4.1 Criteria Used for Animal Classification
You are already familiar with animals — their general characteristics and the enormous diversity found in this group. What you may not yet have seen is how biologists bring order to that diversity in a systematic way.
The animal kingdom contains millions of species, from a sponge fixed to a rock to a blue whale in the open ocean. If we tried to group them by how they look, we would quickly go wrong: a whale looks like a fish, a jellyfish looks like an umbrella, and a tapeworm looks like a ribbon. Appearance alone is a poor guide.
So zoologists agreed on a small set of structural criteria — features of body organisation that are decided early in development and are shared by all members of a group. These criteria are the tools you will use throughout this chapter, so it is worth understanding each one before meeting the phyla.
The criteria used for animal classification are: grades of organisation (cellular, cell-tissue, tissue-organ); body symmetry (asymmetry, radial symmetry, bilateral symmetry); body cavity (acoelomate, pseudocoelomate, coelomate); germ layers (diploblastic, triploblastic); and segmentation (unsegmented, segmented). We will now take these one at a time.
Grades of Organisation
Every animal is multicellular, but not every animal organises its cells to the same degree. The grade of organisation tells us how far the cells of an animal have been arranged into higher units of structure and work.
In the cellular grade, the body is little more than a loose assembly of cells. Each cell carries out most of its own life processes, and there is very little division of labour between them. This is the simplest grade found in the animal kingdom.
In the cell-tissue grade, similar cells have started to group together into definite tissues that perform a shared task — for example, a layer of contractile cells that shortens together. Tissues exist, but they are not yet built up into distinct organs.
In the tissue-organ grade, different tissues combine to form organs, each with a defined job, such as a sucker for attachment or an eyespot for detecting light. As you move further up the animal kingdom, organs themselves become linked into complete organ systems — a digestive system, a nervous system, an excretory system — and this higher level is described as the organ-system grade of organisation. This ladder of grades is summarised in FC 4.1.
Flowchart
FC 4.1 – Grades of organisation: cells (cellular grade) → tissues (cell-tissue grade) → organs (tissue-organ grade) → organ systems (organ-system grade), with complexity and division of labour increasing left to right
The biological meaning of this ladder is division of labour. When every cell must do every job, no cell can specialise. Once tissues form, some cells can be given entirely to contraction, others to digestion, others to conduction of impulses — and each does its own job far better. This is exactly why animals higher on the ladder can be larger, faster and more responsive.
Body Symmetry
Symmetry describes whether the body of an animal can be divided into similar halves, and in how many ways. Look carefully at Fig. 4.26 before reading further, because symmetry is judged by imagining planes cutting through the body.
Diagram
Fig. 4.26 – Types of symmetry: an asymmetrical sponge (no plane of division), a radially symmetrical jellyfish (many vertical planes through the central axis), and a bilaterally symmetrical insect (a single plane giving mirror-image left and right halves), each shown with its cutting planes.
An asymmetrical animal cannot be divided into equal halves by any plane at all. Its body has no regular geometry, which usually goes with a fixed, non-moving way of life.
In radial symmetry, the body parts are arranged around a central axis, like the spokes of a wheel. Any plane passing through that central axis divides the body into two similar halves. Such animals meet their environment equally on all sides.
In bilateral symmetry, only one plane — running from front to back through the midline — divides the body into two mirror-image halves, a left and a right. This kind of animal has a definite front (anterior) and rear (posterior) end.
Bilateral symmetry is far more than a shape. An animal that consistently moves forward meets new surroundings with the same end of its body every time. Sense organs and nerve cells therefore concentrate at that end, forming a head — a process called cephalisation — and this makes the animal a better hunter and a faster responder.
Germ Layers
Very early in development, the embryo of an animal sorts itself into layers of cells called germ layers. Every tissue and organ of the adult body is later built from these layers, so counting them tells us a great deal about how complex the adult can become.
Some animals form only two germ layers: an outer ectoderm and an inner endoderm, with a non-living jelly-like layer lying between them. Such animals are described as diploblastic.
Other animals form a third layer, the mesoderm, between the ectoderm and the endoderm. These are described as triploblastic. The arrangement of both types is compared in Fig. 4.27.
Diagram
Fig. 4.27 – Diploblastic vs triploblastic body: two transverse sections, one showing ectoderm, non-living layer, endoderm and digestive cavity; the other showing ectoderm, mesoderm, endoderm and digestive cavity, with all layers labelled.
The mesoderm is the key to animal complexity. Muscles, blood, the heart, the kidneys, the skeleton and the reproductive organs are all mesodermal in origin. An animal without mesoderm therefore cannot build true muscles or a circulatory system, and this places a firm ceiling on its size and activity.
Body Cavity
The body cavity, or coelom, is a fluid-filled space between the gut and the outer body wall. Whether this space exists, and how it is lined, is one of the most important criteria in animal classification. The three possible conditions are shown side by side in Fig. 4.28.
Diagram
Fig. 4.28 – Three transverse sections comparing an acoelomate (body wall, packed mesodermal tissue, gut), a pseudocoelomate (cavity present but not fully lined by mesoderm) and a coelomate (cavity fully lined by mesoderm on both the body wall and the gut), with all layers labelled.
Acoelomate animals have no body cavity at all. The space between the gut and the body wall is completely packed with mesodermal tissue.
Pseudocoelomate animals have a body cavity, but it is a false one. It is not fully lined by mesoderm — the mesoderm lies only in patches against the body wall and does not cover the gut.
Coelomate animals have a true coelom: a cavity that lies within the mesoderm and is completely lined by it, both on the inner side of the body wall and on the outer surface of the gut.
Why does this matter so much? A true coelom frees the gut from the body wall. The gut can now coil, lengthen and move independently, so digestion is not disturbed every time the animal moves. The cavity fluid also cushions the internal organs, allows them to grow and be arranged in space, and can act as a hydrostatic skeleton against which muscles pull.
Segmentation
The last criterion asks whether the body is built as one continuous unit or as a series of repeating units. In unsegmented animals the body is a single, undivided whole. In segmented animals the body is divided into a linear series of similar parts.
When this repetition is genuine — that is, when the segments repeat not only on the outside but also internally, with muscles, nerves and excretory organs repeated in each unit — it is called metameric segmentation, and each unit is a metamere.
Repeating a working unit many times is a powerful design. Movement becomes finer, because each segment’s muscles can contract slightly out of step with its neighbours, giving smooth waves of motion. Segments can also become specialised for different jobs — some for feeding, some for locomotion — which is how highly specialised body regions such as a head, thorax and abdomen arise.
Taken together, these five criteria form a checklist you can run through for any animal you meet, in the order shown in FC 4.2.
Flowchart
FC 4.2 – Applying the criteria to an unknown animal: is it organised as loose cells, tissues, organs or organ systems? → is it asymmetrical, radial or bilateral? → two germ layers or three? → no cavity, false cavity or true coelom? → unsegmented or segmented? → the phylum follows
How These Criteria Are Tested
These five criteria are the backbone of every question asked on this chapter. In board and NEET-style papers they rarely appear as plain definitions. Instead they appear as match-the-following (phylum against a criterion), as assertion–reason statements such as “Assertion: Ascaris is a pseudocoelomate. Reason: its mesoderm is present only in patches,” and as sequence-ordering questions that ask you to arrange phyla in increasing order of complexity. If you can state, for any animal, its grade of organisation, symmetry, germ layers, body cavity and segmentation, you can answer almost all of them.
4.2 Animal Body Plan
Once we know the criteria used for classification, a second question follows naturally: how is the whole body of an animal laid out? The body plan is the basic architectural design of an animal — how its cells are put together and, above all, how its digestive space opens to the outside. Three body plans are recognised in the animal kingdom.
Cell Aggregate Plan
In this body plan, the cells of the body do not form tissues or organs. There is minimal differentiation among the cells and very little division of labour between them, so the body behaves almost like a colony of cells living together. This plan is found in Porifera, as seen in Fig. 4.1.
Diagram
Fig. 4.1 – Sponge: three vertical sections of increasing complexity showing the body wall, the internal cavity and the direction of water flow through the body, with arrows marking water entering and leaving.
Because no cell hands over its work to another, each cell must feed itself, exchange its own gases and remove its own wastes. This is why an animal built on the cell aggregate plan can never be very active — it has no muscle tissue and no nerve tissue to coordinate movement.
Blind Sac Body Plan
In the blind sac body plan, the body is shaped like a sac with a single opening. Digestion is carried out inside this sac-like structure, and both ingestion (taking food in) and egestion (throwing undigested matter out) take place through the same opening. This plan is seen in the members of Phylum Cnidaria, such as Hydra in Fig. 4.2.
Diagram
Fig. 4.2 – Hydra: longitudinal section of the cylindrical body showing the single opening labelled “Mouth”, the surrounding tentacles and the internal sac-like cavity.
The single opening is the limitation of this design. Food that has just entered and waste that is about to leave share one doorway, so the animal cannot take in a fresh meal while it is still expelling the remains of the last one. Digestion has to happen in batches, and no part of the cavity can specialise for one particular stage of digestion.
Tube Within Tube Body Plan
In this body plan, a digestive system is present in the form of a tube lying within the tube-like body cavity — hence the name. The mouth and the anus are present at two separate ends of the digestive system, as shown in the fish in Fig. 4.3. From Annelida onwards, all phyla show this type of body plan.
Diagram
Fig. 4.3 – Fish shown in outline with the digestive tube running through the body, arrows tracing the one-way path from the mouth at the anterior end to the anus at the posterior end.
The advantage is enormous. Food now travels in one direction only, so different regions of the tube can be given different jobs — grinding, storing, digesting, absorbing, compacting. Feeding also becomes continuous, since a fresh meal can enter the mouth while an earlier one is still being processed further down the tube.
Read in this order, the three body plans form a clear ladder of increasing efficiency, as set out in FC 4.3.
Flowchart
FC 4.3 – Increasing complexity of body plans: cell aggregate plan (no tissues, no gut) → blind sac plan (single opening, digestion in batches) → tube within tube plan (mouth and anus separate, one-way continuous digestion)
4.3 Animal Classification
Armed with the criteria and the body plans, we can now walk through the animal kingdom itself. Kingdom Animalia is divided into eleven phyla, arranged roughly in order of increasing complexity. The first ten are non-chordates, and the eleventh — Chordata — contains the animals that possess a notochord, including ourselves. This overall arrangement is set out in FC 4.4.
Flowchart
FC 4.4 – Kingdom Animalia → Non-chordata: 1. Porifera → 2. Cnidaria → 3. Ctenophora → 4. Platyhelminthes → 5. Aschelminthes → 6. Annelida → 7. Arthropoda → 8. Mollusca → 9. Echinodermata → 10. Hemichordata; and Chordata: 11. Chordata
As you read each phylum, keep a mental checklist running: grade of organisation, symmetry, germ layers, body cavity, segmentation, body plan. Almost every distinguishing statement below is really one of those criteria in action.
1. Phylum: Porifera
The name comes from pori meaning pores and feron meaning bearing — animals that bear pores. Examples include Scypha, Euspongia (bath sponge) and Euplectella (Venus’ flower basket), shown in Fig. 4.4.
Diagram
Fig. 4.4 – Animals of Porifera: (a) Sycon with osculum, ostia, choanocyte, apopyle, body and common base labelled; (b) Euspongia; (c) Euplectella.
These are aquatic animals. Most of them are marine and a few are found in fresh water. They are commonly called sponges.
Most of them have an asymmetrical body. The body of these animals consists of many cells with little division of labour, and for this reason their body is considered a colony of different types of cells rather than a single integrated organism.
These are sedentary animals — they stay fixed in one place. On their body they bear numerous minute pores called ostia, through which water enters the body cavity, the spongocoel. Water leaves the body through a single large opening called the osculum.
The Canal System
Water is circulated in the body through the canal system. During its circulation, the cells of the animal body absorb food, exchange respiratory gases and release excretory products. The whole design is shown in Fig. 4.5.
Diagram
Fig. 4.5 – Typical sponge body: vertical section showing osculum, central cavity, holdfast, epidermis and ostium; enlarged views of a silica spicule, the mesohyl (semi-fluid matrix), and a single choanocyte with collar, flagellum and nucleus.
The spongocoel is lined by special flagellated cells called choanocytes or collar cells. The beating of their flagella creates the water current that drives the whole system, and the complete path taken by that current is summarised in FC 4.5.
Flowchart
FC 4.5 – Water path in a sponge: ostia → canal system → spongocoel → osculum, with choanocyte flagella driving the current and food, gases and wastes exchanged along the way
Support and Reproduction
The body of these animals is supported by calcareous or siliceous spicules, or by proteinaceous spongin fibers. These act as a skeleton, holding the soft body open so that the canals do not collapse.
They reproduce asexually as well as sexually. Asexual reproduction is performed by fragmentation and gemmule formation. These animals also have a strong ability of regeneration.
Sexual reproduction is performed by the formation of gametes. Fertilization is internal, and development takes place through an indirect larval stage — that is, a larva is formed which later changes into the adult.
2. Phylum: Cnidaria
Examples include Hydra, Aurelia (jelly fish), Physalia (Portuguese man-of-war), Adamsia (sea anemone), Diploria (brain coral) and Gorgonia (sea fan).
They are aquatic, mostly marine, and a few are fresh water forms. They may be sessile — fixed in one place — or free swimming.
They show radial symmetry and are diploblastic, with the blind sac body plan. Notice how three of our criteria are being applied in a single sentence.
Polyp and Medusa
These animals exhibit two body forms. The polyp is the cylindrical form, as in Hydra, and the medusa is the umbrella-like form, as in Aurelia, the jelly fish shown in Fig. 4.6.
Diagram
Fig. 4.6 – Aurelia: umbrella-like medusa with umbrellar surface, subumbrellar surface, marginal lappet, marginal tentacles and oral arm labelled.
The body cavity is meant for circulation as well as digestion. Hence it is called the gastrovascular cavity or coelenteron. The name itself records both jobs: “gastro” for digestion and “vascular” for circulation.
Cnidoblasts — The Stinging Cells
The tentacles bear cnidoblasts, or stinging cells, which are meant for anchorage, offence and defence. Their structure before and after discharge is shown in Fig. 4.7.
Diagram
Fig. 4.7 – Cnidoblast before discharge and after discharge: label spines, stylet, refractile rods, lasso, thread tube, operculum, cytoplasm and nucleus in both conditions.
Reproduction and Metagenesis
Cnidarians reproduce both asexually and sexually. Asexual reproduction takes place by budding and regeneration, while sexual reproduction takes place by gamete formation.
They exhibit alternation of the polypoid generation with the medusoid generation. This phenomenon is called metagenesis, that is, polyps produce medusae asexually and medusae produce polyps sexually, as in Obelia. This alternation is traced in FC 4.6.
Flowchart
FC 4.6 – Metagenesis in Obelia: polyp → produces medusa asexually (by budding) → medusa → produces gametes → fertilization → larva → settles and grows into a polyp → back to the start
3. Phylum: Ctenophora
Examples are Pleurobrachia and Ctenoplana. These are commonly called comb jellies or sea walnuts, and Pleurobrachia is shown in Fig. 4.8.
Diagram
Fig. 4.8 – Animals of Ctenophora: Pleurobrachia showing the rows of comb plates; alongside, a colloblast with secretory (adhesive) granules, adhesive granule, radiating fibre, covering cell, spiral filament, straight filament, nucleus, root, synapse, neuron and mesenchyme labelled.
These are exclusively marine, free swimming animals. The body is diploblastic and radially symmetrical, with the blind sac body plan, and the animals have tissue-level organisation.
Locomotion is carried out by eight rows of ciliated comb plates. Rows of cilia beat in sequence and row the animal gently through the water — ctenophores are the largest animals that still swim using cilia.
The characteristic feature of ctenophores is bioluminescence, the ability to produce light. Like cnidarians, ctenophores also exhibit extracellular and intracellular digestion, meaning food is broken down partly in the cavity and partly inside the cells.
Reproduction is sexual, with indirect development. Cnidoblasts are absent, and hence these animals are called acnidarians. Instead, they have colloblasts, which are sticky cells used to capture the prey.
Ctenophora is represented by very few members, and hence it is considered as one of the minor phyla.
4. Phylum: Platyhelminthes
The name comes from platy meaning flat and helminth meaning worms. Examples include Planaria, Taenia (tapeworm) and Fasciola (liver fluke), shown in Fig. 4.9.
Diagram
Fig. 4.9 – Animals of Platyhelminthes: (a) Fasciola with oral sucker, mouth, head lobe and excretory pore; (b) Taenia with neck, sucker, hook, immature proglottid, mature proglottid and gravid proglottid; (c) Planaria with pharynx, gastrovascular cavity, eyespots and ventral nerve cords.
The body of these animals is dorso-ventrally flattened — pressed from top to bottom — hence they are called flat worms. The animals are acoelomate and triploblastic, showing the organ-system grade of organisation.
Most of them are endoparasitic, living inside the body of a host, and a few are free-living. Parasitic forms show the presence of hooks and suckers for attachment to the body of the host.
The body is covered by a cuticle in parasites, or by cilia in free-living forms. The cuticle is a protective layer that resists the host’s digestive juices — without it, a worm sitting in an intestine would simply be digested.
The digestive system is generally absent in parasitic forms, but in free-living forms it is incomplete, following the blind sac plan. A tapeworm living in the host’s intestine is surrounded by digested food, so it can absorb nutrients directly through its body surface and has no need of a gut.
The animals have flame cells, also called protonephridia, which are helpful for excretion and osmoregulation — the regulation of water and salt balance in the body.
The animals are hermaphrodite, that is, bisexual, with both male and female reproductive organs in the same individual. Self fertilization is seen. A few have high power of regeneration and show polyembryony, in which more than one embryo develops from a single fertilized egg.
5. Phylum: Aschelminthes / Nemathelminthes
The names come from ascus meaning sac and helminth meaning worm, and from nema meaning thread. Examples include Ascaris (roundworm), Wuchereria (filarial worm), shown in Fig. 4.10, and Ancylostoma (hook worm).
Diagram
Fig. 4.10 – Wuchereria: the thread-like filarial worm with mouth, sheath, tail and anal pore labelled.
These are mostly parasitic, and a few forms are free-living. The body is long, cylindrical and thread like, and circular in cross-section, hence they are called round worms.
They are triploblastic, bilaterally symmetrical and pseudocoelomate, with the tube within tube body plan. This is the first phylum in our sequence to possess a body cavity of any kind, even though it is a false one.
The body is covered by a tough and resistant cuticle. The body wall has longitudinal muscles but no circular muscles, which is why these worms cannot creep smoothly and instead thrash from side to side in a whip-like motion.
The alimentary canal is complete, with the mouth and anus at opposite ends. The pharynx is well developed and muscular, and acts like a pump that sucks the host’s semi-digested food into the gut.
Excretion takes place by canals or gland cells, and excretory products are eliminated through an excretory pore. The nervous system has a nerve ring and nerves.
Sexual Dimorphism in Ascaris
The animals are unisexual, that is, the sexes are separate. Animals like Ascaris show sexual dimorphism, in which the male and the female of the same species differ visibly in appearance, as shown in Fig. 4.11.
Diagram
Fig. 4.11 – Animals of Aschelminthes: (a) Ascaris male and female showing head, tail, cuticle, mouth with dorsal and ventral lips, cloaca and anus; (b) Ancylostoma hookworm.
Usually the female is longer and broader and has a straight posterior end. The male is shorter and narrower, and has a curved posterior end with a pair of penial setae for copulation.
Fertilization is internal, and development may or may not include a larval stage.
6. Phylum: Annelida
The name comes from annulus, meaning ring. Examples include Nereis, Pheretima (earthworm) and Hirudinaria (leech), shown in Fig. 4.12.
Diagram
Fig. 4.12 – Animals of Annelida: (a) Nereis; (b) Hirudo; (c) Pheretima, showing the ringed, segmented body of each.
They are commonly called ring worms or segmented worms. The animals may be aquatic, and a few may be ectoparasitic or free living or burrowing in moist soil.
They show bilateral symmetry with metameric segmentation — the criterion you met in Section 4.1 now appears for the first time as a defining feature of a phylum. A special region of the body called the clitellum is present.
Locomotion is with the help of longitudinal and circular muscles. Having both sets of muscles is exactly what these animals lacked in the previous phylum, and it allows a segment to be lengthened and shortened in a controlled way. Locomotory structures such as setae (in earthworm), parapodia (in Nereis) or suckers (in leech) are present.
The alimentary canal is complete. Exchange of gases takes place through the body wall. The circulatory system is of the closed type, meaning the blood remains within vessels throughout its journey.
Excretion and osmoregulation are carried out with the help of nephridia. The nervous system consists of a nerve ring and a ventral nerve cord, and the nerve cord is ventral, solid and ganglionated.
Most annelids are hermaphrodites, and a few are dioecious, that is, having separate sexes, as in Nereis.
7. Phylum: Arthropoda
The name comes from arthros meaning joint and podos meaning leg. Examples include cockroach, butterfly, scorpion, millipede and prawn, some of which are shown in Fig. 4.13.
Diagram
Fig. 4.13 – Animals of Arthropoda: (a) Periplaneta; (b) Archispirostreptus; (c) Hottentotta.
This is the largest phylum of kingdom Animalia. These animals have jointed appendages, hence the name Arthropoda.
These are omnipresent — found almost everywhere — and are solitary or colonial. Most of them are free-living, though barnacles are sedentary. A few are parasitic and sanguivorous, meaning blood-feeding, such as the female mosquito and the bed bug.
Their body is bilaterally symmetrical, triploblastic, eucoelomate (possessing a true coelom) and metamerically segmented, with the tube within tube body plan and the organ-system level of organisation.
The Exoskeleton and Moulting
The body is covered by a tough, non-living chitinous exoskeleton. Because this covering is non-living, it cannot grow along with the animal — so they need periodic moulting, called ecdysis, in which the old exoskeleton is shed and a larger one is formed. The body is divided into head, thorax and abdomen.
Organ Systems of Arthropods
The digestive system is complete. The circulatory system is of the open type, in which blood flows through the body cavity, called the haemocoel, rather than remaining in vessels throughout.
Respiratory organs are gills, trachea, book lungs and book gills — a remarkable range, matching the fact that arthropods live in water, on land and in the air.
Excretion takes place by green glands, Malpighian tubules or coxal glands. The nervous system is formed by a nerve ring and a double, ventral, ganglionated nerve cord.
Sense organs are well developed in the form of antennae, simple or compound eyes and various receptors. Sexes are separate, showing sexual dimorphism; fertilization is generally internal; and development is direct or indirect by metamorphosis.
In some arthropods like honey bees and bugs, offsprings are produced by parthenogenesis, that is, development of an unfertilized egg into a new individual. Some insects exhibit polymorphism, the existence of several structurally different forms within one species, for example honey bee, ants and termites.
Economic Importance of Arthropods
Some arthropods are economically important, such as Apis (honey bees) for their honey and wax. Lac is produced by Laccifer lacca, the lac insect. Lobsters, prawns and crabs are edible, and silk worms produce silk.
Some arthropods are harmful and act as vectors, for example the mosquito. Locusta, the locust, is a gregarious pest, moving and feeding in enormous groups. Limulus, the king crab, is known as a living fossil — an organism that has remained almost unchanged over a very long stretch of geological time.
8. Phylum: Mollusca
The name comes from mollis, meaning soft. Examples include Pila, bivalve, Octopus (devil fish), Sepia (cuttle fish), Chaetopleura (chiton), Pinctada (pearl oyster), Loligo (squid), Aplysia (sea hare) and Dentalium (tusk shell), several of which appear in Fig. 4.14.
Diagram
Fig. 4.14 – Animals of Mollusca: (a) Spisula; (b) Chaetopleura; (c) Pila; (d) Sepia; (e) Octopus.
This is the second largest phylum. Molluscs are either free living or sedentary. They are aquatic or seen in marshy places, and a few are terrestrial.
These are soft bodied and show the tube within tube body plan. These are bilaterally symmetrical, but a few are asymmetrical due to torsion, that is, twisting of the body during development.
Body Divisions and the Mantle
The body is divisible into head, foot and visceral mass. The visceral mass, which holds the internal organs, is enclosed in a thick muscular fold of the body wall called the mantle.
The mantle secretes a hard calcareous shell. The shell may be external, as in Pila, internal, as in Sepia, or absent altogether, as in Octopus. A muscular foot is present on the ventral side.
Organ Systems of Molluscs
The digestive system is well developed and complete, with an anterior mouth and a posterior anus. The buccal cavity has a rasping organ called the radula, which is provided with transverse rows of teeth and works like a file scraping food off surfaces.
Aquatic forms show numerous feather-like gills called ctenidia, useful for aquatic respiration. The gills are present in the mantle cavity, which is the space between the visceral mass and the mantle. Terrestrial forms may show the presence of lungs.
The circulatory system is of the open type, except in Sepia, which possesses the closed type. Blood contains a copper-containing blue respiratory pigment called haemocyanin.
Excretion occurs by kidneys, also called the organ of Bojanus. The nervous system is formed by three pairs of ganglia, and the ganglia are interconnected by commissures and connectives.
Sense organs such as eyes for vision, tentacles for tactile sensation, and osphradia for testing the purity of water are present. Sexes are separate, animals are mostly oviparous, and development is direct or indirect.
9. Phylum: Echinodermata
The name comes from echinus meaning spines and derma meaning skin. Examples include Asterias (sea star), Cucumaria (sea cucumber), Echinus (sea urchin), Antedon (sea lily) and Ophiothrix (brittle star), shown in Fig. 4.15.
Diagram
Fig. 4.15 – Animals of Echinodermata: (a) Antedon; (b) Ophiothrix; (c) Asterias; (d) Cucumaria.
These are exclusively marine, solitary, sedentary or free-living and gregarious, and benthic — that is, living on the sea floor.
These are radially symmetrical animals with pentamerous symmetry, in which the body parts are arranged in fives around a central axis. The body may be spherical, elongated or star-shaped.
An important point often missed: the larva of an echinoderm is bilaterally symmetrical, and radial symmetry appears only in the adult. For this reason, echinoderms are grouped with the advanced animals and not with the radially symmetrical Cnidaria.
The endoskeleton is made up of calcareous ossicles. Spines are present on the body, hence the name echinodermata. The body is without definite body divisions; instead, there are two sides, described as oral (bearing the mouth) and aboral (away from the mouth).
The Water Vascular System
The peculiar character of this phylum is the presence of the water vascular system, in which water enters through the madreporite, a sieve-like plate on the body surface. This system is used in locomotion, food capturing, respiration and other functions.
The digestive system is complete. The mouth is ventrally present on the oral surface and the anus on the aboral surface.
Respiration is performed by peristomial gills, papillae, respiratory tree, and so on. The circulatory system and the excretory system are absent — an unusual combination that is frequently tested.
The nervous system is simple, with a nerve ring around the mouth and radial nerves in the arms. Sexes are separate, though sometimes bisexual; fertilization is external; and development is indirect. They show high power of regeneration.
10. Phylum: Hemichordata
The name comes from hemi meaning half and chordata meaning rod. Examples are Balanoglossus and Saccoglossus, shown in Fig. 4.16.
Diagram
Fig. 4.16 – Animals of Hemichordata: (a) Balanoglossus with proboscis, collar, branchial region, genital region, hepatic region and anus labelled; (b) Saccoglossus.
Earlier, this phylum was considered as a sub-phylum of Chordata, because the buccal diverticulum was considered as a notochord. But now it is placed as a separate phylum of Non-chordata.
These are exclusively marine animals, usually living at the bottom of the sea in burrows. Mostly these are free living, but animals like Rhabdopleura are sedentary.
The body is soft, vermiform (worm-like) and unsegmented, and is divided into three parts — proboscis, collar and trunk. The buccal cavity gives rise to a rod-like buccal diverticulum, which is considered as a notochord by some scientists.
The alimentary canal is complete, straight or ‘U’ shaped. Respiration occurs by numerous gills, arranged in two longitudinal rows, present in the pharyngeal region. The gills open by gill slits.
The circulatory system is simple and of the open type. Excretion occurs with the help of the glomerulus. Nervous tissue is embedded in the epidermis on both the dorsal and ventral sides.
The sexes are separate, though sometimes bisexual. Fertilization is external and development is indirect through a free swimming larva. This phylum is the connecting link between non-chordata and chordata.
11. Phylum: Chordata
Chordates are characterised by the presence of a cartilaginous notochord, at least in early embryonic life; the presence of gill slits in the pharyngeal (neck) region; the presence of a hollow, dorsal nerve cord running throughout the length of the body; and a ventral heart.
The phrase “at least in early embryonic life” is the exam-critical part of this definition. You do not have a notochord now, but you did as an embryo — and that is enough to make you a chordate.
Diagram
Fig. 4.29 – Compare and contrast: a generalised non-chordate with exoskeleton, pulsating dorsal blood vessel, ventral nerve cord, digestive tube and anus, drawn above a generalised chordate with notochord, dorsal nerve cord, gill slits, ventral heart, anus and post-anal tail.
Phylum Chordata is divided into three subphyla — Urochordata, Cephalochordata and Vertebrata. Urochordata and Cephalochordata are collectively called Protochordates. The complete hierarchy of the phylum, right down to the classes, is set out in FC 4.7.
Flowchart
FC 4.7 – Phylum Chordata → Subphylum Urochordata and Subphylum Cephalochordata (together called Protochordates), and Subphylum Vertebrata → Division Agnathostomata (Class Cyclostomata) and Division Gnathostomata → Superclass Pisces (Class Chondrichthyes, Class Osteichthyes) and Superclass Tetrapoda (Class Amphibia, Class Reptilia, Class Aves, Class Mammalia)
a. Subphylum: Urochordata or Tunicata
Examples are Herdmania, Salpa and Doliolum, shown in Fig. 4.17.
Diagram
Fig. 4.17 – Animals of Urochordata: (a) Herdmania; (b) Ascidia.
These are also called tunicates or ascidians. They are exclusively marine. The body is soft and covered by a test or tunic, which is made up of tunicine.
The notochord is present only in the tail of the larva, hence the name urochordata — uro meaning tail. The notochord is lost during metamorphosis, so the adult, which sits fixed to a rock, no longer shows the very feature that identifies it as a chordate.
The pharynx has many gill slits. A closed circulatory system is present. Development is indirect.
b. Subphylum: Cephalochordata
The example is Branchiostoma (Amphioxus or lancelet), shown in Fig. 4.18.
Diagram
Fig. 4.18 – Amphioxus: buccal cirri, vestibule, pharynx, gill slits, notochord, dorsal hollow nerve cord, midgut diverticulum, midgut, atriopore, myotomes, anus and caudal fin labelled.
They are exclusively marine. These are also called lancelets, which are small fish like animals that rarely exceed 5 cm in length.
Lancelets partly live buried in soft marine sediments. The notochord extends throughout the length of the body and is present throughout life — which is why the group is named cephalochordata, the notochord reaching right up to the head end.
Myotomes, which are blocks of muscle arranged one after another, are present. A post-anal tail is present. A closed circulatory system is present, and the blood is without pigment.
c. Subphylum: Vertebrata
In these chordates, the notochord is replaced by a cartilaginous or bony vertebral column. This is the decisive advance of the group: a jointed backbone gives rigid support, protects the nerve cord, and offers firm anchorage for large muscles.
Vertebrata is divided into two divisions — Agnathostomata (no jaws) and Gnathostomata (jaws present).
Division 1: Agnathostomata — Class Cyclostomata
This division includes the lowest or most primitive vertebrates, which are without jaws. They include only one class of living vertebrates — the Cyclostomata.
The name Cyclostomata comes from cyclos meaning circular and stoma meaning mouth. Examples are Petromyzon (lamprey), shown in Fig. 4.19, and Myxine (hagfish).
Diagram
Fig. 4.19 – Petromyzon: the eel-like sea lamprey in side view, with an enlarged detail of the circular sucking mouth showing the rings of horny teeth.
Cyclostomes are jawless and eel-like animals. The skin is soft and smooth, containing unicellular mucus glands, but no scales. Median fins are present, but paired fins are absent.
They are ectoparasites. They have a sucking and circular mouth without jaws, and the cranium and vertebral column are made up of cartilage.
The digestive system lacks a stomach. Respiration occurs by 6 to 15 pairs of gill slits, and the gill slits are without an operculum — that is, without a bony gill cover.
The heart is two chambered, with one auricle and one ventricle. The gonad is single, large and without a gonoduct.
Fertilization is external. They are anadromous, that is, they migrate for spawning to fresh water from their marine habitat. After spawning, they die within a few days. The larvae metamorphose and then migrate to the ocean.
Division 2: Gnathostomata
This division is divided into two superclasses — Pisces, which bear fins, and Tetrapoda, which bear four limbs.
Superclass A: Pisces
These are aquatic animals. These are poikilothermic, meaning the body temperature changes according to the change in surrounding temperature.
The lateral line system is present, which shows the presence of rheoreceptors for the detection of water current. This is effectively a sense of touch spread along the whole flank, letting a fish detect movement in water it cannot see.
Locomotion is by body muscles and fins. The caudal fin acts as a steering wheel. The exoskeleton is of dermal scales, and the endoskeleton is either bony or cartilaginous.
The body is streamlined and boat shaped. This feature offers minimum resistance during swimming — a clear structure–function relationship, since water is about eight hundred times denser than air and pushes back hard against any blunt shape.
Respiration is by gills. The heart is two chambered and is ventral in position. It shows single and closed circulation. The heart always shows the presence of deoxygenated blood, so it is described as a venous heart.
They have a well developed brain with large olfactory lobes. Sexes are separate. Most fishes are oviparous and some are viviparous.
Superclass Pisces is divided into two classes as below.
Class 1: Chondrichthyes
The name comes from chondron meaning cartilage and ichthyes meaning fish. Examples include Scoliodon (dog fish), Pristis (sawfish), electric ray, common skate, hammer headed shark, Carcanodon (great white shark) and Trygon (sting ray). Two are shown in Fig. 4.20.
Diagram
Fig. 4.20 – Animals from Chondrichthyes: (a) Scoliodon with eye, mouth, gill clefts, dorsal fin, lateral line, pelvic fin, median ventral fin and heterocercal caudal fin labelled; (b) Anoxypristis.
Chondrichthyes includes the animals in which the endoskeleton is cartilaginous. These are exclusively marine.
The exoskeleton is formed of placoid scales. The teeth are modified placoid scales which are backwardly directed — which is why a shark’s grip tightens as the prey struggles forward.
The mouth is ventral in position. There are 2 pairs of lateral fins, the pectoral and the pelvic. The caudal fin is heterocercal, that is, asymmetrical, with the upper lobe larger than the lower.
Five to seven pairs of gill slits are present. They are not covered by an operculum. The air bladder is absent, hence these fishes need to swim constantly so that they do not sink.
They are predatory fishes. Some of them have electric organs, for example Torpedo, the electric ray, and some have a poison sting, for example Trygon, the sting ray, as organs of offence and defence.
Male copulatory organs called claspers are present. Fertilization is internal. Many of them are viviparous.
Class 2: Osteichthyes
The name comes from osteon meaning bone and ichthyes meaning fish. Examples include Bombay duck, lung fishes (Protopterus, Lepidosiren), Exocoetus (flying fish), Hippocampus (sea-horse), pomphret, Labeo rohita (rohu), Catla (katla), Clarias (magur), and aquarium fishes such as Betta (fighting fish) and Pterophyllum (angle fish). Three are shown in Fig. 4.21.
Diagram
Fig. 4.21 – Animals from Osteichthyes: (a) Catla with eye, mouth, operculum, dorsal fin, pectoral fin, ventral fin, pelvic fin and forked homocercal tail labelled; (b) Exocoetus (flying fish); (c) Hippocampus (sea-horse).
Osteichthyes includes fishes in which a bony endoskeleton is present, hence they are called bony fishes. These are aquatic, present in both fresh and marine waters.
The exoskeleton is formed of cycloid and ctenoid scales. The mouth is mostly terminal in position — at the very tip of the head, rather than underneath.
The tail fin is formed by two equal lobes, that is, it is homocercal or symmetrical. Four pairs of gill slits are present, covered with an operculum.
An air bladder is present to maintain buoyancy, which is why a bony fish can hover motionless in water while a shark must keep swimming. Claspers are absent. Fertilization is external, and these fishes are oviparous.
Superclass B: Tetrapoda
These animals bear two pairs of appendages. Some animals like snakes are secondarily limbless — that is, their ancestors had limbs and the limbs were lost later in evolution.
Superclass Tetrapoda includes four classes, namely Amphibia, Reptilia, Aves and Mammalia.
Class 1: Amphibia
The name comes from amphi meaning both and bios meaning life. Examples include Rana (frog), Bufo (toad), Salamandra (salamander), Ichthyophis (limbless amphibian) and Hyla (tree frog), shown in Fig. 4.22.
Diagram
Fig. 4.22 – Animals from Amphibia: (a) Anura; (b) Bufo; (c) salamander.
Amphibia include the animals which live on land as well as in water — fresh water only. They are poikilothermic animals.
The body is differentiated into head and trunk. The neck and tail are usually absent in many adults, with few exceptions. Two pairs of limbs arise from the pectoral and pelvic girdles respectively, and these help in locomotion.
The skin is moist and glandular, with mucous glands. The exoskeleton is absent. Eyelids are present, and the tympanum represents the ear.
Excretory products, digestive wastes and gametes are released through a common chamber called the cloaca.
The circulatory system is of the closed type. The heart is three chambered and ventral in position. RBCs are biconvex and nucleated.
Respiration is by skin, lungs and buccopharynx. The moist, glandular, exoskeleton-free skin is precisely what makes skin respiration possible — and equally what ties the animal to damp places, since a dry skin cannot exchange gases.
The nervous system is well developed. Sexes are separate. These are oviparous, fertilization is external, and development is indirect through an aquatic larval stage. They show metamorphosis.
Class 2: Reptilia
The name comes from repere, meaning to creep or to crawl. Examples include Naja naja (cobra), Hemidactylus (wall lizard), Chelonia (turtle), Crocodilus (crocodile), Testudo (tortoise), Chameleon (tree lizard), Bangarus (krait) and Vipera (viper), shown in Fig. 4.23.
Diagram
Fig. 4.23 – Animals from Reptilia: (a) lizard; (b) crocodile; (c) tortoise.
Reptilia includes crawling animals. These are the first true terrestrial vertebrates — the first group able to complete its whole life cycle away from water. A few may be aquatic or semi-aquatic, and are also found in marshy areas.
Locomotion occurs by limbs. The limbs are pentadactyl, that is, bearing five digits, and the digits bear claws. Limbs help the animal to walk or creep. Snakes are limbless and crawl on their belly.
Reptiles are poikilotherms. The skin is dry and non-glandular, and is covered by an exoskeleton of epidermal scales or scutes, shields or plates. Lizards and snakes shed their skin periodically.
The tympanum is present. The heart has two complete auricles, but the ventricles are incompletely partitioned. So the heart is not perfectly four chambered, except in the crocodile.
The brain is well developed. The olfactory lobes and the cerebellum are better developed than those of amphibians.
Sexes are separate and show prominent sexual dimorphism. Fertilization is internal. They are oviparous, except the viper, which is viviparous, and they show parental care.
Class 3: Aves
The name comes from avis, meaning bird. Examples include Columba (pigeon), Psittacula (parrot), flightless birds like Struthio (ostrich) and kiwi, Aptenodytes (penguin), Corvus (crow), Neophron (vulture) and Passer (sparrow), shown in Fig. 4.24.
Diagram
Fig. 4.24 – Aves: (a) Struthio; (b) Passer; (c) Psittacoidea; (d) Necrosyrtes; (e) Pavo.
The forelimbs are modified into wings for flying, though some birds have lost the capacity to fly, for example the ostrich. The hind limbs are used for walking, clasping tree branches and running. Aquatic birds have webs between their toes, for example the duck.
The body is streamlined, or boat shaped, to reduce resistance during flight. These are homeotherms, that is, their body temperature remains constant regardless of the surroundings.
The exoskeleton is made up of feathers, and scales are present on the hind limbs — a quiet reminder of the reptilian ancestry of birds.
Adaptations for Flight
The body is differentiated into head, neck, trunk and tail. The skin is thin, dry and non-glandular, except for an oil gland at the base of the tail, called the uropygial gland, whose secretion the bird spreads over its feathers to keep them water-repellent.
Bones are hollow, or pneumatic, with air cavities to reduce body weight. Jaws are without teeth and are modified into a beak — another weight-saving measure, since teeth and heavy jaw bones would load the front of the body.
The crop and gizzard are present in the digestive system. Since there are no teeth to grind food, the muscular gizzard takes over that job inside the body, close to the centre of gravity.
Blood is red in colour due to the presence of red blood cells. RBCs are biconvex and nucleated. The heart is perfectly four chambered, and they show double circulation — a complete separation of oxygenated and deoxygenated blood, which is what sustains the very high energy demand of flight.
Respiration takes place by lungs, having air sacs to increase the buoyancy. The brain is enlarged and has a well developed cerebellum for equilibrium, which a flying animal needs continuously.
Sexes are separate with prominent sexual dimorphism. These are oviparous. Fertilization is internal. Parental care is very well developed, and seasonal migration is seen in some birds.
Class 4: Mammalia
The name comes from mammae, meaning breasts or nipples. Examples include bat, Rattus (rat), Macaca (monkey), Camelus (camel), whale, human being, Cannis (dog), Felis (cat), Elephas (elephant), Equus (horse) and Pteropus (flying fox). Among mammals, Ornithorhynchus (platypus) is oviparous and Macropus (kangaroo) is viviparous. Several are shown in Fig. 4.25.
Diagram
Fig. 4.25 – Mammals: (a) Camelus; (b) Ornithorhynchus; (c) Trachypithecus; (d) Macropus with young in the pouch.
Mammalia includes the animals having mammary glands, that is, milk producing glands, for the nourishment of young ones. This is the defining feature of the entire class.
These are omnipresent. Mostly terrestrial, some are aquatic, and a few are aerial and arboreal, that is, tree-dwelling.
Limbs are the organs of locomotion and are used for walking, flying, climbing, burrowing, swimming, and so on. The body is differentiated into head, neck, trunk and tail.
These are homeotherms. The exoskeleton is in the form of hair, fur, nails, hooves, horns, and so on. The skin is glandular, having sweat glands and sebaceous glands, which are oil glands. Mammary glands are modified sweat glands.
They have an external ear, the pinna. They show heterodont dentition, meaning the teeth of one animal are of several different kinds, each shaped for a different job — cutting, tearing or grinding.
RBCs are biconcave and enucleated, that is, without a nucleus. Blood is red in colour. The heart is ventral in position and four chambered. Respiration takes place by lungs.
The brain is highly developed. The cerebrum shows a transverse band called the corpus callosum, which connects the two halves of the cerebrum.
A few mammals are oviparous, for example the duck billed platypus. Some have pouches for the development of immature young ones, and these are called marsupials, for example the kangaroo. The majority of mammals are placental and viviparous. These three routes are compared in FC 4.8.
Flowchart
FC 4.8 – Three ways mammals produce young: oviparous (Ornithorhynchus) → egg laid → young hatches; marsupial (Macropus) → very immature young born → completes development in pouch; placental (majority) → embryo nourished through placenta → fairly developed young born
4.4 Summary
Animals are classified using a fixed set of structural criteria rather than by outward appearance: grade of organisation, body symmetry, germ layers, body cavity and segmentation.
Grades of organisation form a ladder — cellular, cell-tissue, tissue-organ — rising to the organ-system level in the higher phyla. Each step upward means greater division of labour among cells.
Symmetry may be asymmetrical (no plane of division), radial (many planes through a central axis) or bilateral (a single plane giving mirror-image right and left halves). Bilateral symmetry goes with a definite head end and active, directional movement.
Animals developing from two germ layers, ectoderm and endoderm with a non-living layer between, are diploblastic. Those developing from three layers, with mesoderm added, are triploblastic. Muscles, blood and kidneys are all mesodermal, so only triploblastic animals can build them.
Acoelomates have no body cavity, pseudocoelomates have a cavity not fully lined by mesoderm, and coelomates have a true coelom completely lined by mesoderm.
Segmentation may be absent or present. When both external and internal structures repeat in each unit, it is called metameric segmentation.
Three body plans occur in the animal kingdom: the cell aggregate plan (no tissues or organs, as in Porifera), the blind sac plan (one opening for ingestion and egestion, as in Cnidaria) and the tube within tube plan (separate mouth and anus, seen from Annelida onwards).
Kingdom Animalia is divided into eleven phyla. The first ten are non-chordates and the eleventh is Chordata.
Porifera — sponges; aquatic, mostly marine; asymmetrical; sedentary; water enters through ostia into the spongocoel and leaves by the osculum; circulation through the canal system; spongocoel lined by flagellated choanocytes; supported by calcareous or siliceous spicules or spongin fibers; reproduction by fragmentation and gemmules as well as sexually; internal fertilization and indirect development.
Cnidaria — radially symmetrical, diploblastic, blind sac plan; two body forms, polyp and medusa; gastrovascular cavity or coelenteron serves both digestion and circulation; tentacles bear cnidoblasts for anchorage, offence and defence; metagenesis is the alternation of polypoid and medusoid generations, as in Obelia.
Ctenophora — comb jellies; exclusively marine, free swimming; diploblastic, radially symmetrical, blind sac plan, tissue-level organisation; eight rows of ciliated comb plates for locomotion; bioluminescent; cnidoblasts absent (acnidarians), colloblasts capture prey; a minor phylum.
Platyhelminthes — dorso-ventrally flattened flat worms; acoelomate, triploblastic, organ-system grade; mostly endoparasitic with hooks and suckers; cuticle in parasites, cilia in free-living forms; gut absent in parasites and incomplete in free-living forms; flame cells or protonephridia for excretion and osmoregulation; hermaphrodite with self fertilization; regeneration and polyembryony in a few.
Aschelminthes (Nemathelminthes) — round worms; triploblastic, bilaterally symmetrical, pseudocoelomate, tube within tube; tough cuticle; only longitudinal muscles; complete alimentary canal with muscular pharynx; excretion by canals or gland cells; unisexual with sexual dimorphism in Ascaris; internal fertilization.
Annelida — segmented worms with metameric segmentation and a clitellum; setae, parapodia or suckers for locomotion; complete alimentary canal; gas exchange through body wall; closed circulation; nephridia for excretion and osmoregulation; solid, ventral, ganglionated nerve cord; mostly hermaphrodite, Nereis dioecious.
All animals from Annelida onwards are triploblastic and coelomate, with the organ-system level of organisation.
Arthropoda — the largest phylum; jointed appendages; eucoelomate and metamerically segmented; chitinous non-living exoskeleton requiring moulting (ecdysis); body of head, thorax and abdomen; open circulation through the haemocoel; respiration by gills, trachea, book lungs or book gills; excretion by green glands, Malpighian tubules or coxal glands; parthenogenesis and polymorphism in some insects; many economically important species.
Mollusca — the second largest phylum; soft bodied, tube within tube; body of head, foot and visceral mass; mantle secretes a calcareous shell that may be external, internal or absent; radula in the buccal cavity; ctenidia in the mantle cavity; open circulation except in Sepia; blue copper pigment haemocyanin; excretion by the organ of Bojanus; osphradia test the purity of water.
Echinodermata — exclusively marine and benthic; adults radially symmetrical with pentamerous symmetry; endoskeleton of calcareous ossicles; oral and aboral surfaces; water vascular system entered through the madreporite serves locomotion, food capture and respiration; circulatory and excretory systems absent; external fertilization, indirect development and high regeneration.
Hemichordata — exclusively marine, burrowing; body of proboscis, collar and trunk; buccal diverticulum once mistaken for a notochord; gill slits in the pharyngeal region; open circulation; excretion by glomerulus; now placed in Non-chordata and regarded as the connecting link between non-chordates and chordates.
Chordata — characterised by a cartilaginous notochord at least in early embryonic life, pharyngeal gill slits, a hollow dorsal nerve cord and a ventral heart; divided into Urochordata, Cephalochordata and Vertebrata, of which the first two are the Protochordates.
Urochordata — tunicates; body covered by a test of tunicine; notochord confined to the larval tail and lost at metamorphosis; closed circulation. Cephalochordata — lancelets under 5 cm; notochord throughout the body and throughout life; myotomes and post-anal tail present; blood without pigment.
Vertebrata — notochord replaced by a cartilaginous or bony vertebral column; divided into Agnathostomata (jawless) and Gnathostomata (jawed).
Cyclostomata — the only living class of Agnathostomata; jawless, eel-like ectoparasites; no scales and no paired fins; cartilaginous cranium and vertebral column; no stomach; 6 to 15 pairs of gill slits without operculum; two chambered heart; anadromous, dying soon after spawning.
Pisces — aquatic, poikilothermic; lateral line system with rheoreceptors; streamlined body; gills for respiration; two chambered venous heart with single, closed circulation. Chondrichthyes have a cartilaginous skeleton, placoid scales, ventral mouth, heterocercal tail, no operculum and no air bladder, with claspers and internal fertilization. Osteichthyes have a bony skeleton, cycloid and ctenoid scales, terminal mouth, homocercal tail, operculum and an air bladder, with external fertilization.
Amphibia — live on land and in fresh water; poikilothermic; moist glandular skin without exoskeleton; tympanum and cloaca present; closed circulation with a three chambered heart; respiration by skin, lungs and buccopharynx; external fertilization and indirect development with metamorphosis.
Reptilia — first true terrestrial vertebrates; poikilothermic; dry, non-glandular skin with epidermal scales, scutes or plates; pentadactyl limbs with claws, snakes secondarily limbless; ventricles incompletely partitioned except in the crocodile; internal fertilization; oviparous except the viper, with parental care.
Aves — homeotherms; forelimbs modified into wings; feathers as exoskeleton; pneumatic bones, toothless beak, crop and gizzard; four chambered heart with double circulation; lungs with air sacs; urinary bladder absent and only the left ovary and oviduct present, both reducing body weight; oviparous with well developed parental care and seasonal migration in some.
Mammalia — defined by mammary glands, which are modified sweat glands; homeotherms; hair, fur, nails, hooves or horns as exoskeleton; glandular skin with sweat and sebaceous glands; pinna present; heterodont dentition; enucleated biconcave RBCs; four chambered heart; corpus callosum in the cerebrum; a few oviparous, some marsupial, the majority placental and viviparous.
The whole chapter can be read as one rising sequence of complexity, condensed in FC 4.9.
Flowchart
FC 4.9 – Rising complexity across the animal kingdom: cellular grade and no symmetry (Porifera) → tissue grade, radial, diploblastic, blind sac (Cnidaria, Ctenophora) → organ-system grade, bilateral, triploblastic, acoelomate (Platyhelminthes) → pseudocoelomate with tube within tube (Aschelminthes) → true coelom with metameric segmentation (Annelida) → jointed appendages and exoskeleton (Arthropoda) → mantle and shell (Mollusca) → water vascular system (Echinodermata) → gill slits and buccal diverticulum (Hemichordata) → notochord, dorsal hollow nerve cord and ventral heart (Chordata) → vertebral column (Vertebrata) → jaws (Gnathostomata) → four limbs (Tetrapoda) → constant body temperature (Aves, Mammalia)
4.5 Points to Ponder
The body of a sponge is described as a colony of different types of cells rather than as a single organised animal. At what point does a group of cells stop being a colony and start being one animal?
Cnidarians are radially symmetrical throughout life, while an echinoderm larva is bilaterally symmetrical and only the adult becomes radial. If symmetry can change within a single lifetime, how reliable is it as a criterion of classification?
A tapeworm has no digestive system at all, and the roundworm has lost its circular muscles. Losing a structure is usually called degeneration — but is losing something always a step backwards for a parasite?
The pseudocoelom of a roundworm is called a false body cavity, yet roundworms are among the most numerous animals on earth. Does a “higher” body design always mean a more successful animal?
Arthropods must shed their entire exoskeleton in order to grow, and lie defenceless while the new one hardens. Why has a design with such an obvious cost become the largest phylum in the animal kingdom?
Most molluscs have an open circulatory system, but Sepia has a closed one. What kind of way of life might make a closed system worth the extra structural cost?
Hemichordata was once treated as a subphylum of Chordata and is now placed among the non-chordates. What does such a change tell you about the nature of a classification system itself?
Birds and mammals are both homeotherms, but they arrived at this condition along separate lines. Can two animals share a striking feature without being closely related, and how would a biologist tell the difference?
Cnidaria and Ctenophora are both diploblastic, radially symmetrical and built on the blind sac plan, yet one is a large phylum and the other is a minor one with very few members. What might decide whether a body design flourishes or barely survives?
Echinoderms have no circulatory system and no excretory system, and yet they are placed high in the animal kingdom. Should “advanced” be measured by how many organ systems an animal possesses?
A chordate is defined by features that may exist only in early embryonic life and vanish long before adulthood. Is it reasonable to classify an animal by structures that an observer can never see in the living adult?
Suppose an unfamiliar animal is brought up from a deep-sea trench. Which of the criteria you have studied would you test first, and what would you be able to conclude — and not conclude — from the outside of its body alone?
4.6 Check Your Concepts
Why is bilateral symmetry almost always found together with a distinct head end, while radially symmetrical animals have no head at all?
Why can an animal with a blind sac body plan not develop a stomach and an intestine?
Why is the mesoderm described as the germ layer that made large, active animals possible?
Why is a flatworm able to survive without any circulatory system, even though it is triploblastic and has an organ-system grade of organisation?
Why does an arthropod have to moult, while a mollusc with a hard shell never does?
Why do the eggs and skin of amphibians restrict them to moist habitats, while reptiles could colonise dry land?
Why does a bird need a four chambered heart and double circulation more urgently than a reptile does?
Why is metagenesis useful to a cnidarian that spends most of its life fixed to a rock?
Why is Hemichordata no longer placed within Phylum Chordata, even though it possesses pharyngeal gill slits?
What would happen to a sponge if the flagella of all its choanocytes stopped beating?
What would happen to an earthworm if its circular muscles were absent and only longitudinal muscles remained?
What would happen to a bony fish if its air bladder were removed?
What would happen to a female bird if it developed both ovaries and both oviducts?
What would happen to a sea star if its madreporite were blocked?
Arrange the following in the correct order of increasing complexity of body cavity, and name one animal for each: coelomate, acoelomate, pseudocoelomate.
Arrange in the correct sequence the path taken by water through the body of a sponge: canal system, osculum, ostia, spongocoel.
Arrange the following taxa of Phylum Chordata in the correct order from the largest group to the smallest: Class Mammalia, Division Gnathostomata, Phylum Chordata, Subphylum Vertebrata, Superclass Tetrapoda.
Arrange the stages of metagenesis in Obelia in the correct cyclic order: gamete formation, medusa formed by budding, larva, polyp.
Draw a labelled transverse section of a coelomate animal and mark the ectoderm, mesoderm, endoderm, gut and coelom. Then redraw the same section to show the pseudocoelomate condition.
Draw a single choanocyte and label the collar, flagellum and nucleus. Mark with arrows the direction in which water moves past the cell.
Draw a cnidoblast before and after discharge and label the operculum, thread tube, stylet and nucleus in each condition.
Distinguish between a cnidoblast and a colloblast, giving one animal that possesses each.
Distinguish between an open and a closed circulatory system, and name one phylum that shows each.
Distinguish between haemoglobin and haemocyanin on the basis of the metal present, the colour produced, and the group of animals in which each is found.
Distinguish between a polyp and a medusa on the basis of shape, mode of life and mode of reproduction.
Distinguish between dermal and epidermal scales, giving one animal that possesses each.
An unfamiliar marine animal is found to be triploblastic, bilaterally symmetrical, metamerically segmented and coelomate, with a closed circulatory system and nephridia. To which phylum does it most likely belong, and which single feature was most decisive in your answer?
A student examines an animal and reports that it has a soft body, a muscular foot, a rasping radula and blue blood, but no shell. Name the phylum, and explain how an animal of that phylum can lack a shell.
Two aquatic animals are brought to the laboratory. One has a heterocercal tail, ventral mouth and no operculum; the other has a homocercal tail, terminal mouth and an operculum. Identify the class of each and state which one is likely to be viviparous.
A biologist studying an embryo notices a notochord, a hollow dorsal nerve cord, pharyngeal gill slits and a post-anal tail, but the adult form of the same animal shows none of these. Is the animal a chordate? Justify your answer using the definition of Chordata.
Reptiles are known for having a three chambered heart. Which animal shows a near four chambered condition in reptiles?
The circulatory system has evolved from the open to the closed type in the animal kingdom. Which phylum can be called the first to represent closed circulation?
Pinna is part of the external ear and it is found in mammals. Do aves and reptiles show an external ear in any form?
Fish and frog can respire in water. Can they respire through their skin? If yes, why do they have gills?
Birds need to keep their body light to help in flying. Hence they show the presence of some organs only on one side. How does their skeleton help in reducing their weight?
Cnidarians and ctenophorans are both diploblastic. Which other character do they have in common, which is not found in other phyla?
Crab and snail both have a protective covering. Is it made up of the same material?
Sponge and sea star show calcareous protective material. Do they belong to the same phylum?
Fish and snake both have scales. How do these scales differ from each other?
Lower phyla like Arthropoda and Cnidaria show metamorphosis. Is it also found in any class of Phylum Chordata?
Draw a neat labelled diagram of Sycon.
Draw a neat labelled diagram of Aurelia.
Draw a neat labelled diagram of Amphioxus.
Draw a neat labelled diagram of Catla.
Draw a neat labelled diagram of Balanoglossus.
Draw a neat labelled diagram of Scoliodon.
Identify the animals given in the pictures (A to I) and write the features of the phylum or class to which each belongs.
Diagram
Fig. 4.30 – Exercise plate of nine unlabelled animal photographs marked A to I: a flatworm, a limbless amphibian, a dolphin, a snake, a sea urchin, a lizard, a crab-like arthropod, a chambered molluscan shell, and a lancelet-like chordate.
- Observe the animals given below and identify the body symmetry of each.
Diagram
Fig. 4.31 – Exercise plate showing three animals side by side with symmetry planes drawn through each: a sponge, a jelly fish and an insect.
4.7 Objective Questions
- Which of the following belongs to a minor phylum?
- Comb jelly
- Jelly fish
- Herdmania
- Salpa
- Select the animal having a venous heart.
- Crocodile
- Salamander
- Rohu
- Toad
- In Ascaris, __________.
- mesoglea is present
- endoderm is a discontinuous layer
- mesoderm is present in patches
- body cavity is absent
- Which of the following is incorrect in the case of birds?
- Presence of teeth
- Presence of scales
- Nucleated RBCs
- Hollow bones
- Chitinous exoskeleton is a characteristic of __________.
- Dentalium
- Antedon
- Millipede
- Sea urchin
For each of the following, state whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: A sponge has no digestive, respiratory or excretory organs. Reason: Water flowing through the canal system reaches every cell of its body.
Assertion: A tapeworm has no alimentary canal. Reason: The tapeworm lives surrounded by the digested food of its host.
Assertion: A shark must keep swimming throughout its life. Reason: Chondrichthyes have a cartilaginous endoskeleton.
Assertion: The heart of a fish is described as a venous heart. Reason: Blood passes through the heart only once in each complete circuit of the body.
Assertion: The adult Herdmania is still classified as a chordate. Reason: The notochord is present in the tail of its larva.
Assertion: A diploblastic animal can never possess a true coelom. Reason: A true coelom is a cavity formed within the mesoderm and lined by it.
Match the phylum in the first column with the correct character in the second column.
| Phylum | Characters |
|---|---|
| a. Annelida | i. Tube feet |
| b. Mollusca | ii. Ostia |
| c. Ctenophora | iii. Radula |
| d. Porifera | iv. Parapodia |
| e. Echinodermata | v. Comb plates |
- Match the structure in the first column with its function in the second column.
| Structure | Function |
|---|---|
| a. Madreporite | i. Excretion and osmoregulation in flatworms |
| b. Osphradium | ii. Testing the purity of water |
| c. Flame cell | iii. Entry of water into the water vascular system |
| d. Uropygial gland | iv. Rasping food from a surface |
| e. Radula | v. Oiling the feathers |
- Match the animal in the first column with the group it represents in the second column.
| Animal | Group |
|---|---|
| a. Limulus | i. Marsupial mammal |
| b. Macropus | ii. Living fossil |
| c. Ornithorhynchus | iii. Jawless vertebrate |
| d. Petromyzon | iv. Oviparous mammal |
| e. Obelia | v. Shows metagenesis |