Animal Kingdom

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, that is, 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, that is, 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, 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

(2) closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).

Figure 4.1 summary: This figure consists of two illustrative diagrams. The first illustration depicts an organism with a central axis and appendages extending outward, while the second illustration shows a crab divided by a single central line. The figure demonstrates the difference between radial symmetry, where parts are arranged around a central point, and bilateral symmetry, where the body can be divided into two mirroring halves. It can be inferred that radial symmetry is characteristic of organisms with a circular body plan, whereas bilateral symmetry is typical of organisms with a distinct left and right side.

Figure 4.2 summary: This figure consists of two schematic diagrams. The diagrams illustrate the organization of germinal layers in different biological structures, specifically comparing diploblastic and triploblastic arrangements. The first diagram shows a structure with two primary cellular layers, the ectoderm and endoderm, separated by a non-cellular mesoglea. The second diagram depicts a structure with three cellular layers, adding a mesoderm between the ectoderm and endoderm. It can be inferred that triploblastic organisms possess a more complex tissue organization than diploblastic ones due to the presence of an additional embryonic germ layer.

4.1.2 Symmetry

Animals can be categorised on the basis of their symmetry. Sponges are mostly asymmetrical, that is, 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, etcetera, 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, for example, coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm (Figure 4.2a).

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).

4.1.4 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, for example, 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, for example, aschelminths (Figure 4.3b). The animals in which the body cavity is absent are called acoelomates, for example, platyhelminthes (Figure 4.3c).

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.

Figure 4.3 summary: This figure consists of three diagrammatic sectional views. The illustrations represent the anatomical body cavity arrangements of different organisms, specifically showing coelomate, pseudocoelomate, and acoelomate structures. By comparing the three diagrams, it can be inferred that coelomates possess a true body cavity fully lined by mesoderm, pseudocoelomates have a body cavity that is only partially lined by mesoderm, and acoelomates lack a body cavity entirely, with the space between the gut and the body wall filled with tissue.

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, for example, 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.

Figure 4.4 summary: This figure is a taxonomic flow chart. It illustrates the hierarchical classification of the Kingdom Animalia by branching from general characteristics to specific phyla. The classification process is based on a sequence of fundamental biological features, including levels of organization, body symmetry, and the presence or type of body cavity. The chart shows that animals are first divided by their organizational complexity, then by their symmetry, and finally by their coelom type. From this structure, it can be inferred that the most complex phyla, such as Chordata and Arthropoda, share bilateral symmetry and a true coelom, while simpler phyla like Porifera are characterized by cellular-level organization and a lack of a body cavity. Additionally, the classification indicates that symmetry and body cavity types serve as primary evolutionary markers for distinguishing between different animal groups.

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.

Figure 4.5 summary: This figure consists of a series of biological illustrations. The content displays three different examples of organisms from the phylum Porifera, specifically identifying Sycon, Euspongia, and Spongilla. Based on the illustrations, it can be inferred that Porifera exhibit significant morphological diversity, ranging from elongated tubular forms and rounded mass shapes to irregular structures with numerous finger-like projections.

Digestion is intracellular. The body is supported by a skeleton made up of spicules or spongin fibres. Sexes are not separate (hermaphrodite), that is, 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.

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 cnidoblasts or ncidocytes (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.

Figure 4.6 summary: This figure consists of a set of biological illustrations. The content displays two different examples of Coelenterata, showing both detailed anatomical drawings and simplified body outlines. One example represents Aurelia in the medusa form, and the other represents Adamsia in the polyp form. From these illustrations, it can be inferred that Coelenterata exhibit distinct morphological variations, with the medusa form characterized by an umbrella-like shape and the polyp form characterized by a cylindrical body with tentacles extending from the top.

Figure 4.7 summary: This figure is a diagrammatic illustration. It depicts the internal structure of a cnidoblast, showcasing the specialized stinging cell including the nematocyst capsule and its associated trigger mechanism. The illustration demonstrates that the cnidoblast contains a complex organelle designed for rapid discharge, indicating its function as a defensive or predatory tool used by certain aquatic organisms to capture prey or deter threats.

Some of the cnidarians, for example, 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, etcetera whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish. Those cnidarians which exist in both forms exhibit alternation of generations (Metagenesis), that is, 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).

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.

Figure 4.8 summary: This figure is a biological illustration. It depicts a specimen of Ctenophora, specifically the species Pleurobrachia, showcasing its spherical body structure and distinctive anatomical features. The illustration highlights the presence of longitudinal ciliary combs used for locomotion and the elongated tentacles extending from the body. Based on the morphology shown, it can be inferred that the organism is adapted for a planktonic existence, utilizing its specialized comb rows for swimming and its tentacles for capturing prey.

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.

Figure 4.9 summary: This figure consists of two biological illustrations. The content displays two different examples of flatworms, specifically a tapeworm and a liver fluke. Based on the illustrations, it can be inferred that members of the Platyhelminthes phylum exhibit diverse body plans, ranging from the elongated, segmented structure of the tapeworm to the leaf-like, unsegmented shape of the liver fluke.

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).

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.

Figure 4.10 summary: This figure consists of biological illustrations. The top portion displays the morphology of male and female roundworms, representing the phylum Aschelminthes, while the bottom portion shows examples of annelids, including a segmented worm and a leech. The illustrations highlight the distinct structural differences between the phyla, where roundworms exhibit a smooth, non-segmented cylindrical body, whereas annelids are characterized by clear body segmentation and specialized appendages in some species.

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), that is, 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).

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.

Figure 4.12 summary: This figure consists of a series of biological illustrations. The content displays four different animals: a locust, a scorpion, a butterfly, and a prawn. These images serve as representative examples of the phylum Arthropoda, highlighting the diverse morphological forms found within this group, such as those with wings, pincers, or segmented exoskeletons.

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.

Figure 4.13 summary: This figure consists of two biological illustrations. The image displays representative examples of the phylum Mollusca, specifically showing a snail and an octopus. These illustrations highlight the diverse morphological forms within the mollusk group, contrasting a gastropod with a protective hard shell against a cephalopod with a soft body and multiple tentacles. This comparison demonstrates the wide range of structural adaptations found among different classes of mollusks.

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.

They are usually dioecious and oviparous with indirect development.

Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Seahare), 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.

Figure 4.14 summary: This figure consists of two biological illustrations. The images display two different examples of organisms from the phylum Echinodermata, specifically showing a starfish and a brittle star. The first illustration depicts a species with thick, fleshy arms, while the second illustration shows a species with significantly thinner, more flexible arms. These examples highlight the morphological diversity within the phylum, contrasting the robust body structure of Asterias with the slender, elongated appendages of Ophiura.

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 stomachord, 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.

Figure 4.15 summary: This figure is a biological diagram. It illustrates the anatomical structure of a Balanoglossus, labeling the primary body regions. The diagram identifies three distinct sections: the anterior proboscis, the middle collar, and the posterior trunk. Based on the illustration, the organism exhibits a segmented body plan where the proboscis is the most forward extension, followed by a collar that transitions into a longer, curved trunk.

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 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.

Figure 4.16 summary: This figure is a biological diagram. It illustrates the anatomical characteristics of the phylum Chordata, labeling key structural features including the nerve cord, the notochord, gill slits, and the post-anal part. The diagram demonstrates that chordates are defined by a specific set of morphological traits, such as the presence of a supportive notochord and a dorsal nerve cord, alongside specialized respiratory openings and an extension of the body beyond the anus.

Table 4.1 presents a comparison of salient features of chordates and non-chordates.

Table 4.1 summary: This table outlines the primary anatomical differences between chordates and non-chordates, highlighting that chordates possess a notochord, a dorsal hollow central nervous system, pharyngeal gill slits, and a post-anal tail, whereas non-chordates lack these features or possess contrasting structures, such as a ventral solid central nervous system and a dorsal heart.

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.

Figure 4.17 summary: This figure is a biological illustration. It depicts the external anatomy of an Ascidia, showing its elongated, sac-like body structure with distinct openings at the top and bottom. The illustration highlights the overall morphology of the organism, including its textured surface and the specific positioning of its siphons. Based on the depiction, it can be inferred that the organism possesses a simplified body plan characteristic of tunicates, designed for filter feeding within its environment.

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.

Image summary: This figure is a hierarchical classification chart. It illustrates the taxonomic division of the subphylum Vertebrata, branching from the main group into Agnatha and Gnathostomata based on the presence or absence of a jaw. Gnathostomata is further categorized into the super classes of Pisces and Tetrapoda, distinguished by whether they possess fins or limbs. The chart concludes by listing the specific classes under each group, including Cyclostomata for the jawless group, Chondrichthyes and Osteichthyes for the fish, and Amphibia, Reptilia, Aves, and Mammals for the tetrapods. The diagram demonstrates that vertebrates are broadly split by jaw development, with jawed vertebrates further diversifying into aquatic and land-dwelling forms.

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 to 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.

Figure 4.18 summary: This figure is a biological illustration. It depicts the external anatomy of Petromyzon, showcasing a streamlined, elongated body with a rounded snout, small eyes, and a series of gill slits located behind the head. The illustration reveals a lack of paired fins and a jawless mouth structure. Based on these anatomical features, it can be inferred that this organism is a primitive vertebrate adapted for an aquatic environment, characterized by a simplified skeletal structure typical of jawless fish.

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).

Figure 4.19 summary: This figure consists of two biological illustrations. The images depict two different types of cartilaginous fishes, specifically showing a side profile of a Scoliodon and a top-down view of a Pristis. The illustrations highlight the distinct morphological differences between the two species, such as the streamlined body and typical shark-like fins of the Scoliodon compared to the elongated, saw-like snout and flattened body structure of the Pristis. It can be inferred that these organisms represent the diversity in body forms within the class of cartilaginous fishes, adapting to different ecological niches and hunting strategies.

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.

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, that is, 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.

Figure 4.20 summary: This figure consists of two illustrative drawings. The image displays two different species of bony fishes, specifically a seahorse and a catla fish. The comparison highlights the significant morphological diversity within bony fishes, showing a contrast between the specialized, curved body structure of the seahorse and the more conventional, streamlined body shape of the catla.

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.

Figure 4.21 summary: This figure consists of two illustrative diagrams. The images depict two different examples of animals belonging to the class Amphibia, specifically showing a salamander and a frog. Based on the visual representations, the figure demonstrates the morphological diversity within amphibians, contrasting the elongated body and tail of the salamander with the shorter, more compact body structure of the frog.

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 (Limless amphibia).

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.

Figure 4.22 summary: This figure is a collection of biological illustrations. The content consists of four separate drawings depicting different types of reptiles, specifically a chameleon, a crocodile, a turtle, and a cobra. These illustrations demonstrate the diverse morphological characteristics within the reptile class, ranging from the specialized gripping limbs and prehensile tail of the chameleon to the protective shell of the turtle, the armored skin of the crocodile, and the limbless body of the snake.

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 lizard), 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.

Figure 4.23 summary: This figure consists of a series of illustrative images. The content displays four different bird species, identified as Neophron, Struthio, Psittacula, and Pavo. Based on the visual evidence, the figure demonstrates the significant morphological diversity among birds, showing variations in body size, beak shape, leg length, and plumage characteristics across different avian genera.

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, that is, they are able to maintain a constant body temperature. Respiration is by 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 mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw.

Figure 4.24 summary: This figure consists of a series of biological illustrations. The content displays four different mammalian species: a platypus, a kangaroo, a fruit bat, and a whale. These examples demonstrate the vast morphological diversity within the class Mammalia, showcasing animals adapted for diverse environments including aquatic, terrestrial, and aerial habitats.

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.

Table 4.2 summary: This table outlines the comparative anatomical and physiological characteristics across various animal phyla, illustrating a trend of increasing biological complexity. It shows a progression from cellular and tissue levels of organization in simpler phyla to complex organ systems in more advanced groups. Key evolutionary developments are highlighted, including the transition from radial to bilateral symmetry, the emergence of a coelom, and the gradual appearance of specialized circulatory and respiratory systems. The data demonstrates that while primitive phyla lack most complex systems, higher phyla consistently possess complete digestive systems and integrated internal transport and gas exchange mechanisms.

Summary

The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etcetera, 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 platyhelminthes 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.

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:

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

Table summary: The table lists various anatomical features and their corresponding animal phyla or classes, pairing specific biological structures with the taxonomic groups they characterize.

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