46 BIOLOGY

When you look around, you will observe different animals with different

structures and forms. As over a million species of animals have been

described till now, the need for classification becomes all the more

important. The classification also helps in assigning a systematic position

to newly described species.

4.1 BASIS OF CLASSIFICATION

Inspite of differences in structure and form of different animals, there are

fundamental features common to various individuals in relation to the

arrangement of cells, body symmetry, nature of coelom, patterns of

digestive, circulatory or reproductive systems. These features are used

as the basis of animal classification and some of them are discussed here.

4.1.1 Levels of Organisation

Though all members of Animalia are multicellular, all of them do not

exhibit the same pattern of organisation of cells. For example, in sponges,

the cells are arranged as loose cell aggregates, i.e., they exhibit cellular

level of organisation. Some division of labour (activities) occur among

the cells. In coelenterates, the arrangement of cells is more complex. Here

the cells performing the same function are arranged into tissues, hence is

called tissue level of organisation. A still higher level of organisation, i.e.,

organ level is exhibited by members of Platyhelminthes and other higher

phyla where tissues are grouped together to form organs, each specialised

for a particular function. In animals like Annelids, Arthropods, Molluscs,

NIMAL

INGDOM

HAPTER

4.1 Basis of

Classification

4.2 Classification of

Animals

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47ANIMAL KINGDOM

Echinoderms and Chordates, organs have

associated to form functional systems, each

system concerned with a specific physiological

function. This pattern is called organ system

level of organisation. Organ systems in different

groups of animals exhibit various patterns of

complexities. For example, the digestive system

in Platyhelminthes has only a single opening

to the outside of the body that serves as both

mouth and anus, and is hence called

incomplete. A complete digestive system has

two openings, mouth and anus. Similarly, the

circulatory system may be of two types:

(i) open type in which the blood is pumped

out of the heart and the cells and tissues are

directly bathed in it and

(ii) closed type in which the blood is circulated

through a series of vessels of varying diameters

(arteries, veins and capillaries).

4.1.2 Symmetry

Animals can be categorised on the basis of their

symmetry. Sponges are mostly asymmetrical,

i.e., any plane that passes through the centre

does not divide them into equal halves. When

any plane passing through the central axis of

the body divides the organism into two identical

halves, it is called radial symmetry.

Coelenterates, ctenophores and echinoderms

have this kind of body plan (Figure 4.1a).

Animals like annelids, arthropods, etc., where

the body can be divided into identical left and

right halves in only one plane, exhibit bilateral

symmetry (Figure 4.1b).

4.1.3 Diploblastic and Triploblastic

Organisation

Animals in which the cells are arranged in two

embryonic layers, an external ectoderm and

an internal endoderm, are called diploblastic

animals, e.g., coelenterates. An undifferentiated

layer, mesoglea, is present in between the

ectoderm and the endoderm (Figure 4.2a).

Figure 4.2 Showing germinal layers :

(a) Diploblastic (b) Triploblastic

(a)

(b)

Ectoderm

Mesoglea

Endoder

Mesoder

Figure 4.1 (b) Bilateral symmetry

Figure 4.1 (a) Radial symmetry

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48 BIOLOGY

4.1.4 Coelom

Presence or absence of a cavity between the body

wall and the gut wall is very important in

classification. The body cavity, which is lined

by mesoderm is called coelom. Animals

possessing coelom are called coelomates, e.g.,

annelids, molluscs, arthropods, echinoderms,

hemichordates and chordates (Figure 4.3a). In

some animals, the body cavity is not lined by

mesoderm, instead, the mesoderm is present as

scattered pouches in between the ectoderm and

endoderm. Such a body cavity is called

pseudocoelom and the animals possessing them

are called pseudocoelomates, e.g.,

aschelminthes (Figure 4.3b). The animals in

which the body cavity is absent are called

acoelomates, e.g., platyhelminthes (Figure 4.3c).

Figure 4.3 Diagrammatic sectional view of :

(a) Coelomate (b) Pseudocoelomate

Those animals in which the developing embryo has a third germinal layer,

mesoderm, in between the ectoderm and endoderm, are called

triploblastic animals (platyhelminthes to chordates, Figure 4.2b).

4.1.5 Segmentation

In some animals, the body is externally and internally divided into

segments with a serial repetition of at least some organs. For example, in

earthworm, the body shows this pattern called metameric segmentation

and the phenomenon is known as metamerism.

4.1.6 Notochord

Notochord is a mesodermally derived rod-like structure formed on the

dorsal side during embryonic development in some animals. Animals with

notochord are called chordates and those animals which do not form this

structure are called non-chordates, e.g., porifera to echinoderms.

4.2 CLASSIFICATION OF ANIMALS

The broad classification of Animalia based on common fundamental

features as mentioned in the preceding sections is given in Figure 4.4.

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ANIMAL KINGDOM

49ANIMAL KINGDOM

The important characteristic features of the

different phyla are described.

4.2.1 Phylum – Porifera

Members of this phylum are commonly known

as sponges. They are generally marine and mostly

asymmetrical animals (Figure 4.5). These are

primitive multicellular animals and have cellular

level of organisation. Sponges have a water

transport or canal system. Water enters through

minute pores (ostia) in the body wall into a central

cavity, spongocoel, from where it goes out

through the osculum. This pathway of water

transport is helpful in food gathering, respiratory

exchange and removal of waste. Choanocytes

or collar cells line the spongocoel and the canals.

Digestion is intracellular. The body is supported

by a skeleton made up of spicules or spongin

fibres. Sexes are not separate (hermaphrodite),

i.e., eggs and sperms are produced by the same

individual. Sponges reproduce asexually by

fragmentation and sexually by formation of

gametes. Fertilisation is internal and development

is indirect having a larval stage which is

morphologically distinct from the adult.

*Echinodermata exhibits radial or bilateral symmetry depending on the stage.

Figure 4.4 Broad classification of Kingdom Animalia based on common fundamental features

(a)

(b)

(c)

Figure 4.5 Examples of Porifera : (a) Sycon

(b) Euspongia (c) Spongilla

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50 BIOLOGY

cnidoblasts or cnidocytes (which contain the stinging capsules or

nematocysts) present on the tentacles and the body. Cnidoblasts are used

for anchorage, defense and for the capture of prey (Figure 4.7). Cnidarians

exhibit tissue level of organisation and are diploblastic. They have a central

gastro-vascular cavity with a single opening, mouth on hypostome.

Digestion is extracellular and intracellular. Some of the cnidarians, e.g.,

corals have a skeleton composed of calcium carbonate. Cnidarians exhibit

two basic body forms called polyp and medusa (Figure 4.6). The former

is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the

latter is umbrella-shaped and free-swimming like

Aurelia or jelly fish.

Those cnidarians which exist in both forms exhibit alternation of

generation (Metagenesis), i.e., polyps produce medusae asexually and

medusae form the polyps sexually (e.g., Obelia).

Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone),

Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

Figure 4.7

Diagrammatic view of

Cnidoblast

Figure 4.6 Examples of Coelenterata indicating outline of their body form :

(a) Aurelia (Medusa) (b) Adamsia (Polyp)

(b)

(a)

Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia

(Bath sponge).

4.2.2 Phylum – Coelenterata (Cnidaria)

They are aquatic, mostly marine, sessile or free-swimming, radially

symmetrical animals (Figure 4.6). The name cnidaria is derived from the

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51ANIMAL KINGDOM

4.2.3 Phylum – Ctenophora

Ctenophores, commonly known as sea walnuts or comb jellies

are exclusively marine, radially symmetrical, diploblastic

organisms with tissue level of organisation. The body bears

eight external rows of ciliated comb plates, which help in

locomotion (Figure 4.8). Digestion is both extracellular and

intracellular. Bioluminescence (the property of a living

organism to emit light) is well-marked in ctenophores. Sexes

are not separate. Reproduction takes place only by sexual

means. Fertilisation is external with indirect development.

Examples:

Pleurobrachia and Ctenoplana.

4.2.4 Phylum – Platyhelminthes

They have dorso-ventrally flattened body, hence are called

flatworms (Figure 4.9). These are mostly endoparasites found

in animals including human beings. Flatworms are bilaterally

symmetrical, triploblastic and acoelomate animals with organ

level of organisation. Hooks and suckers are present in the

parasitic forms. Some of them absorb nutrients from the host

directly through their body surface. Specialised cells called

flame cells help in osmoregulation and excretion. Sexes are not

separate. Fertilisation is internal and development is through

many larval stages. Some members like Planaria possess high

regeneration capacity.

Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

Figure 4.8 Example of

Ctenophora

(Pleurobrachia)

(a)

(b)

Figure 4.9 Examples of Platyhelminthes : (a) Tape worm (b) Liver fluke

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52 BIOLOGY

4.2.5 Phylum – Aschelminthes

The body of the aschelminthes is circular in

cross-section, hence, the name roundworms

(Figure 4.10). They may be freeliving, aquatic

and terrestrial or parasitic in plants and animals.

Roundworms have organ-system level of body

organisation. They are bilaterally symmetrical,

triploblastic and pseudocoelomate animals.

Alimentary canal is complete with a well-

developed muscular pharynx. An excretory

tube removes body wastes from the body cavity

through the excretory pore. Sexes are separate

(dioecious), i.e., males and females are distinct.

Often females are longer than males. Fertilisation

is internal and development may be direct (the

young ones resemble the adult) or indirect.

Examples : Ascaris (Roundworm), Wuchereria

(Filaria worm), Ancylostoma (Hookworm).

4.2.6 Phylum – Annelida

They may be aquatic (marine and fresh water) or

terrestrial; free-living, and sometimes parasitic.

They exhibit organ-system level of body

organisation and bilateral symmetry. They are

triploblastic, metamerically segmented and

coelomate animals. Their body surface is

distinctly marked out into segments or

metameres and, hence, the phylum name

Annelida (Latin, annulus : little ring) (Figure 4.11).

They possess longitudinal and circular muscles

which help in locomotion. Aquatic annelids like

Nereis

possess lateral appendages, parapodia,

which help in swimming. A closed circulatory

system is present. Nephridia (sing. nephridium)

help in osmoregulation and excretion. Neural

system consists of paired ganglia (sing. ganglion)

connected by lateral nerves to a double ventral

nerve cord. Nereis, an aquatic form, is dioecious,

but earthworms and leeches are monoecious.

Reproduction is sexual.

Examples : Nereis, Pheretima (Earthworm) and

Hirudinaria (Blood sucking leech).

Figure 4.11 Examples of Annelida : (a) Nereis

(b) Hirudinaria

Male Female

Figure 4.10 Example of

Aschelminthes:

Roundworm

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53ANIMAL KINGDOM

4.2.7 Phylum – Arthropoda

This is the largest phylum of Animalia which

includes insects. Over two-thirds of all named

species on earth are arthropods (Figure 4.12).

They have organ-system level of organisation.

They are bilaterally symmetrical, triploblastic,

segmented and coelomate animals. The body

of arthropods is covered by chitinous

exoskeleton. The body consists of head, thorax

and abdomen. They have jointed appendages

(arthros-joint, poda-appendages). Respiratory

organs are gills, book gills, book lungs or

tracheal system. Circulatory system is of open

type. Sensory organs like antennae, eyes

(compound and simple), statocysts or

balancing organs are present. Excretion takes

place through malpighian tubules. They are

mostly dioecious. Fertilisation is usually

internal. They are mostly oviparous.

Development may be direct or indirect.

Examples: Economically important insects –

Apis (Honey bee), Bombyx (Silkworm), Laccifer

(Lac insect)

Vectors – Anopheles, Culex and Aedes

(Mosquitoes)

Gregarious pest – Locusta (Locust)

Living fossil – Limulus (King crab).

4.2.8 Phylum – Mollusca

This is the second largest animal phylum

(Figure 4.13). Molluscs are terrestrial or aquatic

(marine or fresh water) having an organ-system

level of organisation. They are bilaterally

symmetrical, triploblastic and coelomate

animals. Body is covered by a calcareous shell

and is unsegmented with a distinct head,

muscular foot and visceral hump. A soft and

spongy layer of skin forms a mantle over the

visceral hump. The space between the hump

and the mantle is called the mantle cavity in

which feather like gills are present. They have

respiratory and excretory functions. The

anterior head region has sensory tentacles. The

mouth contains a file-like rasping organ for

feeding, called radula.

Figure 4.12 Examples of Arthropoda :

(a) Locust (b) Butterfly

(a)

(c)

(b)

(d)

Figure 4.13 Examples of Mollusca :

(a) Pila (b) Octopus

(b)

(a)

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54 BIOLOGY

They are usually dioecious and oviparous with indirect

development.

Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia

(Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-

hare), Dentalium (Tusk shell) and Chaetopleura (Chiton).

4.2.9 Phylum – Echinodermata

These animals have an endoskeleton of calcareous ossicles

and, hence, the name Echinodermata (Spiny bodied, Figure

4.14). All are marine with organ-system level of organisation.

The adult echinoderms are radially symmetrical but larvae

are bilaterally symmetrical. They are triploblastic and

coelomate animals. Digestive system is complete with mouth

on the lower (ventral) side and anus on the upper (dorsal)

side. The most distinctive feature of echinoderms is the

presence of water vascular system which helps in

locomotion, capture and transport of food and respiration.

An excretory system is absent. Sexes are separate.

Reproduction is sexual. Fertilisation is usually external.

Development is indirect with free-swimming larva.

Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon

(Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).

4.2.10 Phylum – Hemichordata

Hemichordata was earlier considered as a sub-phylum under

phylum Chordata. But now it is placed as a separate phylum

under non-chordata. Hemichordates have a rudimentary

structure in the collar region called stomochord, a structure

similar to notochord.

This phylum consists of a small group of worm-like

marine animals with organ-system level of organisation. They

are bilaterally symmetrical, triploblastic and coelomate

animals. The body is cylindrical and is composed of an

anterior proboscis, a collar and a long trunk (Figure 4.15).

Circulatory system is of open type. Respiration takes place

through gills. Excretory organ is proboscis gland. Sexes are

separate. Fertilisation is external. Development is indirect.

Examples: Balanoglossus and Saccoglossus.

4.2.11 Phylum – Chordata

Animals belonging to phylum Chordata are fundamentally

characterised by the presence of a notochord, a dorsal

Figure 4.14 Examples of

Echinodermata :

(a) Asterias

(b) Ophiura

(a)

(b)

Figure 4.15 Balanoglossus

Proboscis

Collar

Trunk

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ANIMAL KINGDOM

55ANIMAL KINGDOM

hollow nerve cord and paired pharyngeal

gill slits (Figure 4.16). These are bilaterally

symmetrical, triploblastic, coelomate with

organ-system level of organisation. They

possess a post anal tail and a closed circulatory

system.

Table 4.1 presents a comparison of salient

features of chordates and non-chordates.

Phylum Chordata is divided into three

subphyla: Urochordata or Tunicata,

Cephalochordata and Vertebrata.

Subphyla Urochordata and

Cephalochordata are often referred to as

protochordates (Figure 4.17) and are

exclusively marine. In Urochordata, notochord

is present only in larval tail, while in

Cephalochordata, it extends from head to tail

region and is persistent throughout their life.

Examples: Urochordata – Ascidia, Salpa,

Doliolum; Cephalochordata – Branchiostoma

(Amphioxus or Lancelet).

The members of subphylum Vertebrata

possess notochord during the embryonic

period. The notochord is replaced by a

cartilaginous or bony vertebral column in the

adult. Thus all vertebrates are chordates but

all chordates are not vertebrates. Besides the

basic chordate characters, vertebrates have a

ventral muscular heart with two, three or four

chambers, kidneys for excretion and

osmoregulation and paired appendages which

may be fins or limbs.

Nerve cord

Notochord

Post-anal part

Gill slits

Figure 4.16 Chordata characteristics

Figure 4.17 Ascidia

TABLE 4.1 Comparison of Chordates and Non-chordates

S.No. Chordates Non-chordates

1. Notochord present. Notochord absent.

2. Central nervous system is dorsal, Central nervous system is ventral, solid

hollow and single. and double.

3. Pharynx perforated by gill slits. Gill slits are absent.

4. Heart is ventral. Heart is dorsal (if present).

5. A post-anal part (tail) is present. Post-anal tail is absent.

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56 BIOLOGY

Figure 4.18 A jawless vertebrate - Petromyzon

Figure 4.19 Example of Cartilaginous fishes :

(a) Scoliodon (b) Pristis

(a)

(b)

4.2.11.1 Class – Cyclostomata

All living members of the class Cyclostomata are

ectoparasites on some fishes. They have an

elongated body bearing 6-15 pairs of gill slits

for respiration. Cyclostomes have a sucking and

circular mouth without jaws (Fig. 4.18). Their

body is devoid of scales and paired fins.

Cranium and vertebral column are

cartilaginous. Circulation is of closed type.

Cyclostomes are marine but migrate for

spawning to fresh water. After spawning, within

a few days, they die. Their larvae, after

metamorphosis, return to the ocean.

Examples: Petromyzon (Lamprey) and Myxine

(Hagfish).

4.2.11.2 Class – Chondrichthyes

They are marine animals with streamlined body

and have cartilaginous endoskeleton

(Figure 4.19). Mouth is located ventrally.

Notochord is persistent

throughout life. Gill

slits are separate and without operculum (gill

cover). The skin is tough, containing minute

placoid scales. Teeth are modified placoid

scales which are backwardly directed. Their

jaws are very powerful. These animals are

predaceous. Due to the absence of air bladder,

they have to swim constantly to avoid sinking.

Vertebrata

Division

Agnatha

(lacks jaw)

Class

1. Cyclostomata

Gnathostomata

(bears jaw)

Super Class

Pisces

(bear fins)

Tetrapoda

(bear limbs)

Class

1. Amphibia

2. Reptilia

3. Aves

4. Mammals

Class

1. Chondrichthyes

2. Osteichthyes

The subphylum Vertebrata is further divided as follows:

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ANIMAL KINGDOM

57ANIMAL KINGDOM

Heart is two-chambered (one auricle and one ventricle).

Some of them have electric organs (e.g., Torpedo) and

some possess poison sting (e.g., Trygon). They are

cold-blooded (poikilothermous) animals, i.e., they lack

the capacity to regulate their body temperature. Sexes

are separate. In males pelvic fins bear claspers. They

have internal fertilisation and many of them are

viviparous.

Examples: Scoliodon (Dog fish), Pristis (Saw fish),

Carcharodon (Great white shark), Trygon (Sting ray).

4.2.11.3 Class – Osteichthyes

It includes both marine and fresh water fishes with bony

endoskeleton. Their body is streamlined. Mouth is

mostly terminal (Figure 4.20). They have four pairs of

gills which are covered by an operculum on each side.

Skin is covered with cycloid/ctenoid scales. Air bladder

is present which regulates buoyancy. Heart is two-

chambered (one auricle and one ventricle). They are

cold-blooded animals. Sexes are separate. Fertilisation

is usually external. They are mostly oviparous and

development is direct.

Examples: Marine – Exocoetus (Flying fish),

Hippocampus

(Sea horse); Freshwater – Labeo (Rohu),

Catla (Katla), Clarias (Magur); Aquarium – Betta

(Fighting fish), Pterophyllum (Angel fish).

4.2.11.4 Class – Amphibia

As the name indicates (Gr., Amphi : dual, bios, life),

amphibians can live in aquatic as well as terrestrial

habitats (Figure 4.21). Most of them have two pairs of

limbs. Body is divisible into head and trunk. Tail may

be present in some. The amphibian skin is moist

(without scales). The eyes have eyelids. A tympanum

represents the ear. Alimentary canal, urinary and

reproductive tracts open into a common chamber called

cloaca which opens to the exterior. Respiration is by

gills, lungs and through skin. The heart is three-

chambered (two auricles and one ventricle). These are

cold-blooded animals. Sexes are separate. Fertilisation

is external. They are oviparous and development

is indirect.

Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog),

Salamandra (Salamander), Ichthyophis (Limbless

amphibia).

Figure 4.21 Examples of Amphibia :

(a) Salamandra

(b) Rana

(a)

(b)

Figure 4.20 Examples of Bony fishes :

(a) Hippocampus (b) Catla

(b)

(a)

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58 BIOLOGY

4.2.11.5 Class – Reptilia

The class name refers to their creeping or crawling mode of locomotion

(Latin, repere or reptum, to creep or crawl). They are mostly terrestrial

animals and their body is covered by dry and cornified skin, epidermal

scales or scutes (Fig. 4.22). They do not have external ear openings.

Tympanum represents ear. Limbs, when present, are two pairs. Heart is

usually three-chambered, but four-chambered in crocodiles. Reptiles are

poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes

are separate. Fertilisation is internal. They are oviparous and development

is direct.

Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizar

d),

Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator).

Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus

(Krait), Vipera (Viper).

4.2.11.6 Class – Aves

The characteristic features of Aves (birds) are the presence of feathers

and most of them can fly except flightless birds (e.g., Ostrich). They possess

beak (Figure 4.23). The forelimbs are modified into wings. The hind limbs

generally have scales and are modified for walking, swimming or clasping

the tree branches. Skin is dry without glands except the oil gland at the

base of the tail. Endoskeleton is fully ossified (bony) and the long bones

are hollow with air cavities (pneumatic). The digestive tract of birds has

additional chambers, the crop and gizzard. Heart is completely four-

chambered. They are warm-blooded (homoiothermous) animals, i.e.,

they are able to maintain a constant body temperature. Respiration is by

Figure 4.22 Reptiles: (a) Chameleon (b) Crocodilus (c) Chelone (d) Naja

(b)

(c)

(a)

(d)

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59ANIMAL KINGDOM

lungs. Air sacs connected to lungs supplement respiration. Sexes are

separate. Fertilisation is internal. They are oviparous and development is

direct.

Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio

(Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

4.2.11.7 Class – Mammalia

They are found in a variety of habitats – polar ice caps, deserts, mountains,

forests, grasslands and dark caves. Some of them have adapted to fly or

live in water. The most unique mammalian characteristic is the presence

of milk producing glands (mammary glands) by which the young ones

are nourished. They have two pairs of limbs, adapted for walking, running,

climbing, burrowing, swimming or flying (Figure 4.24). The skin of

(a)

Figure 4.23 Some birds : (a) Neophron (b) Struthio (c) Psittacula (d) Pavo

(b)

(c)

(d)

Figure 4.24 Some mammals : (a) Ornithorhynchus (b) Macropus (c) Pteropus (d) Balaenoptera

(a)

(b)

(c)

(d)

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60 BIOLOGY

mammals is unique in possessing hair. External ears or pinnae are

present. Different types of teeth are present in the jaw. Heart is four-

chambered. They are homoiothermous. Respiration is by lungs. Sexes

are separate and fertilisation is internal. They are viviparous with few

exceptions and development is direct.

Examples: Oviparous-Ornithorhynchus (Platypus); Viviparous -

Macropus (Kangaroo),

Pteropus (Flying fox), Camelus (Camel), Macaca

(Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant),

Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale),

Panthera tigris (Tiger), Panthera leo (Lion).

The salient distinguishing features of all phyla under animal kingdom

is comprehensively given in the Table 4.2.

Level of

Organi-

sation

Cellular

Tissue

Organ &

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Symme-

try

Various

Radial

Bilateral

Radial

Bilateral

Coelom

Absent

Pseudo

coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Segmen-

tation

Absent

Present

Absent

Present

Digestive

System

Absent

Incomplete

Complete

Circu-

latory

System

Absent

Present

Respi-

ratory

System

Absent

Present

Distinctive

Features

Body with pores

and canals in walls.

Cnidoblasts

present.

Comb plates for

locomotion.

Flat body, suckers.

Often worm-

shaped,

elongated.

Body segment-

ation like rings.

Exoskeleton of cu-

ticle, jointed ap-

pendages.

External skeleton

of shell usually

present.

Water vascular

system, radial

symmetry.

Worm-like with

proboscis, collar

and trunk.

Notochord, dorsal

hollow nerve cord,

gill slits with

limbs or fins.

Phylum

Porifera

Coelenterata

(Cnidaria)

Ctenophora

Platyhelm-

inthes

Aschelmin-

thes

Annelida

Arthropoda

Mollusca

Echino-

dermata

Hemi-

chordata

Chordata

TABLE 4.2 Salient Features of Different Phyla in the Animal Kingdom

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ANIMAL KINGDOM

61ANIMAL KINGDOM

SUMMARY

The basic fundamental features such as level of organisation, symmetry, cell

organisation, coelom, segmentation, notochord, etc., have enabled us to broadly

classify the animal kingdom. Besides the fundamental features, there are many

other distinctive characters which are specific for each phyla or class.

Porifera includes multicellular animals which exhibit cellular level of

organisation and have characteristic flagellated choanocytes. The coelenterates

have tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating.

The ctenophores are marine animals with comb plates. The platyhelminths have

flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers

and hooks. Aschelminthes are pseudocoelomates and include parasitic as well as

non-parasitic roundworms.

Annelids are metamerically segmented animals with a true coelom. The

arthropods are the most abundant group of animals characterised by the presence

of jointed appendages. The molluscs have a soft body surrounded by an external

calcareous shell. The body is covered with external skeleton made of chitin. The

echinoderms possess a spiny skin. Their most distinctive feature is the presence

of water vascular system. The hemichordates are a small group of worm-like marine

animals. They have a cylindrical body with proboscis, collar and trunk.

Phylum Chordata includes animals which possess a notochord either

throughout or during early embryonic life. Other common features observed in

the chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits.

Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess

jaws (Gnathostomata). Agnatha is represented by the class, Cyclostomata. They

are the most primitive chordates and are ectoparasites on fishes. Gnathostomata

has two super classes, Pisces and Tetrapoda. Classes Chondrichthyes and

Osteichthyes bear fins for locomotion and are grouped under Pisces. The

Chondrichthyes are fishes with cartilaginous endoskeleton and are marine. Classes,

Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thus

grouped under Tetrapoda. The amphibians have adapted to live both on land and

water. Reptiles are characterised by the presence of dry and cornified skin. Limbs

are absent in snakes. Fishes, amphibians and reptiles are poikilothermous (cold-

blooded). Aves are warm-blooded animals with feathers on their bodies and

forelimbs modified into wings for flying. Hind limbs are adapted for walking,

swimming, perching or clasping. The unique features of mammals are the presence

of mammary glands and hairs on the skin. They commonly exhibit viviparity.

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62 BIOLOGY

EXERCISES

1. What are the difficulties that you would face in classification of animals, if common

fundamental features are not taken into account?

2. If you are given a specimen, what are the steps that you would follow to classify

it?

3. How useful is the study of the nature of body cavity and coelom in the

classification of animals?

4. Distinguish between intracellular and extracellular digestion?

5. What is the difference between direct and indirect development?

6. What are the peculiar features that you find in parasitic platyhelminthes?

7. What are the reasons that you can think of for the arthropods to constitute the

largest group of the animal kingdom?

8. Water vascular system is the characteristic of which group of the following:

(a) Porifera (b) Ctenophora (c) Echinodermata (d) Chordata

9. “All vertebrates are chordates but all chordates are not vertebrates”. Justify the

statement.

10. How important is the presence of air bladder in Pisces?

11. What are the modifications that are observed in birds that help them fly?

12. Could the number of eggs or young ones produced by an oviparous and viviparous

mother be equal? Why?

13. Segmentation in the body is first observed in which of the following:

(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda

14. Match the following:

(a) Operculum (i) Ctenophora

(b) Parapodia (ii) Mollusca

(d) Comb plates (iv) Reptilia

(e) Radula (v) Annelida

(f) Hairs (vi) Cyclostomata and Chondrichthyes

(g) Choanocytes (vii) Mammalia

(h) Gill slits (viii) Osteichthyes

15. Prepare a list of some animals that are found parasitic on human beings.

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