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Animals are multicellular, heterotrophic, eukaryotes who exhibit different patterns of organisation of several types of cells.

Basis of Classification

The fundamental features common to various individuals that are used as the basis of animal classification have been given below

•

Levels of Organisation

Though all the members of kingdom-Animalia are multicellular, yet all of them do not exhibit the same pattern of cellular organisation. Different levels of organisation are discussed below. 

■. Cellular level (cell aggregates) found in sponges. 

■. 

Tissue level

(cell performing same function are arranged into tissues) found in coelenterates and ctenophores.

■. 

Organ level

(tissues grouped together to form organs) found in phylum-Platyhelminthes and other higher phyla.

■. Organ system level (association of organs to form functional systems) found in annelids to chordates.

■. Symmetry On the basis of symmetry, animals can be asymmetrical, i.e. body cannot be divided into equal halves by any plane (e.g. sponges), radially symmetrical, L.e. body can be divided into equal halves by any plane passing through the central axis (e.g. coelenterates, ctenophores and echinoderms) and bilaterally symmetrical, i.e. body can be divided into two identical halves (left and right) only along one plane (e.g. annelids and arthropods).

■. Diploblastic and Triploblastic Organisation On the basis of germ layers, animals are classified as Diploblastic (cells arranged in two embryonic layers, i.e. external ectoderm and internal endoderm), e.g. coelenterates and triploblastic (cells arranged in three germ layers, i.e. ectoderm and endoderm and an undifferentiated layer mesoderm between them), e.g. phylum-Platyhelminthes to Chordata.

• Coelom It is the body cavity (present between body wall and gut wall), which is lined by mesoderm. Animals are also classified on the basis of the presence or absence of coelom as given below. 

Coelomates Animals which possess coelom. They are further classified into schizocoelomates (e.g. annelids, molluscs and arthropods) and enterocoelomates (e.g. echinoderms and chordates).

■ Acoelomates The animal in which body cavity is absent are called acoelomates, e.g. Porifera to Platyhelminthes

(true acoelomates).

• Pseudocoelomates Mesoderm does not line the body cavity and is present between ectoderm and endoderm as scattered pouches, e.g. Aschelminthes.

• Segmentation In some animals, the body is externally and internally divided into segments or somites with a serial repetition of at least some organs. In earthworm, this phenomenon is known as metamerism.

⚫ Notochord It 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 without notochord are non-chordates, e.g. Porifera to Echinodermata.

Classification of Animals

Broadly kingdom-Animalia is classified as follows

I. Non-chordates

The non-chordates include the following phyla

1. Phylum-Porifera includes sponges, which are usually marine and mostly asymmetrical animals with canal system as most important features.

⚫ Central cavity present in sponges is known as spongocoel, it is lined by collar cells or choanocytes and it opens to outside by osculum.

• Water enters the spongocoel through minute pores called ostia and moves out through osculum.

⚫ The body is supported by spicules or spongin and protein fibres, which form skeletal system.

. They are hermaphrodites, i.e. both male and female

gametes are produced within same individual.

Fertilisation is internal and development is indirect.

Sponges reproduce asexually by fragmentation and sexually by gamete formation. Larval forms are morphologically distinct form adults, e.g. Sycon, Spongilla and Euspongia.

2. Phylum-Cnidaria (Coelenterata) consists of aquatic marine, sessile, radially symmetrical animals. round

⚫ Tentacles are either present over the mouth or around their body edges. Cells called cnidoblasts or cnidocytes are present on the tentacles and the body. These cells are used for anchorage, defence and capture of prey.

• A central gastrovascular cavity (coelenteron) with a single opening, mouth on hypostome is present.

. Some cnidarians, e.g. corals, have skeleton composed of CaCO3.

They show polymorphism with two basic body plans, i.e. polyps are fixed, sessile, cylindrical, e.g. Hydra, Adamsia, etc., and medusae are umbrella-shaped and free-swimming, e.g. Aurelia.

. The cnidarians exist in both forms and exhibit alternation of generation (metagenesis), i.e. polyps produce medusae asexually and medusae produce polyps sexually (e.g. Obelia).

3. Phylum-Ctenophora (Comb jellies or Sea walnuts) • These are exclusively marine, diploblastic, radially symmetrical, acoelomate organisms with tissue level of organisation.

Body is soft, transparent and gelatinous with well-marked bioluminescence (the property of a living organism to emit light).

⚫ Eight external rows or ciliated comb plates help in

locomotion. • Digestion is both extracellular and intracellular.

⚫ These are sexually reproducing, monoecious organisms with external fertilisation and indirect development, e.g. Ctenophora and Pleurobrachia.

4. Phylum-Platyhelminthes (Flatworms) ⚫ These are dorsoventrally flattened, bilaterally

symmetrical, triploblastic and acoelomate animals. They are mostly endoparasites and show organ system level of organisation.

⚫ They have specialised cells for excretion and osmoregulation called flame cells.

• They possess a high regeneration capacity. Fertilisation is internal and development is through many larval stages, e.g. Planaria, Taenia (tapeworm) and Fasciola (liver fluke).

5. Phylum-Aschelminthes (Roundworms)

⚫ They are bilaterally symmetrical, triploblastic and

pseudocoelomate animals with the body being circular in cross-section. 

⚫ They are free-living, aquatic, terrestrial or parasitic forms.

They are dioecious (separate sexes) and show internal fertilisation with indirect development. Females are often longer than males.

• Alimentary canal is complete with a well-developed muscular pharynx.

• Excretion is through excretory pore. Fertilisation is internal and development may be direct or indirect. e.g. Ascaris (roundworm), Wuchereria (filaria worm) and Ancylostoma (hookworm).

6. Phylum-Annelida (Segmented worms) They are triploblastic show organ level of body organisation and are bilaterally symmetrical.

⚫ They show metameric segmentation, i.e. body surface is distinctly marked out into segments or metameres. • Locomotion is aided by longitudinal and circular

muscles. In Nereis, swimming is achieved by lateral

appendages called parapodia. • Respiration is through skin or gills, circulatory system is closed and digestive system is complete.

• Excretion is through nephridia. Both monoecious, e.g. Nereis and dioecious forms, e.g. Pheretima (earthworm) and Hirudinaria (leech) occur.

• Neural system consists of paired ganglia connected by lateral nerve to a double ventral nerve cord.

• They reproduce sexually.

7. Phylum-Arthropoda Largest phylum of kingdom- Animalia, includes insects.

⚫ They are triploblastic, segmented, bilaterally symmetrical coelomate animals. Body is covered by chitinous exoskeleton.Body consists of head, thorax and abdomen. 

They possess jointed appendages. 

⚫ Circulatory system is open type, forming a haemocoel.

• Sensory organs like antennae, eyes, statocysts or balancing organs are present.

• Respiratory system shows diverse range, e.g. gills, trachea, book lungs, general body surface and book gills, e.g. Apis, Culex, Limulus (a living fossil), etc.

• Excretion takes place through Malpighian tubules. 

⚫ Fertilisation is internal with direct or indirect development. Mostly dioecious.

8. Phylum-Mollusca It is the second largest phylum.

⚫ These are terrestrial or aquatic, mostly marine and some are freshwater.. . These are bilaterally symmetrical, triploblastic and coelomate animals.

⚫ Body is unsegmented and covered by a calcareous shell but consists of a distinct head, muscular foot and visceral hump. The space between hump and mantle is called mantle cavity in which feather like gills are present. cavity in which feather like

• They have respiratory and excretory functions. The anterior head region has sensory tentacles. ⚫ Feeding is performed through radula. Circulation is open type, excretion through organ of Bojanus or metanephridia pair.

• Sexes are separate and are mostly oviparous, e.g. Octopus, Pila, Sepia.

⚫ They are usually dioecious and oviparous with indirect development.

9. Phylum-Echinodermata have an endoskeleton of calcareous ossicles.

• They are radially (pentamerous) symmetrical at adult stage and bilaterally symmetrical at larval stage.

⚫ They are triploblastic and coelomate animals.

• Water vascular system is present, which helps in locomotion, capture and transport of food and respiration.

• Complete digestive system is present and an

excretory system is absent.

• Sexes are separate and reproduction is sexual with indirect development and free-swimming larvae, e.g. Asterias (starfish), Echinus (sea urchin), Cucumaria (sea cucumber).

10. Phylum-Hemichordata (Half chordates)

⚫ These are bilaterally symmetrical, tripoblastic and

coelomate worm-like marine animals. 

⚫ Body is cylindrical and divided into proboscis, collar and trunk. Notochord is absent.

• Excretion occurs through proboscis gland, circulation is open type and respiration occurs through gill slit pairs.

• Sexes are separate, fertilisation is external and development is indirect.

• Connecting link between echinoderms and chordates, . e.g. Balanoglossus, etc.

II. Phylum-Chordata

These have notochord, dorsal hollow nerve chord, paired pharyngeal gill slits and post-anal tail at some stages of life. They are divided into following subphyla

1. Urochordata (tunicates) Notochord is present only in larval tail, e.g. Herdmania.

2. Cephalochordata (lancelets) Notochord persists throughout life and extends from head to tail, e.g. Branchiostoma.

Note Urochordata and Cephalochordata are often referred to as protochordata.

3. Vertebrata (vertebrates) Notochord replaced by vertebral column in adults.

Some Major Classes of Vertebrates Characteristic features of some major classes of subphylum- Vertebrata are as follows

1. Class-Cyclostomata

• These are ectoparasites on some fishes, have 6-15 pairs of gill slits. Sucking and circular mouth without jaws. These possess sucktorial tongue that bears horny teeth.

• Cranium and vertebral column are cartilaginous.

• They die after spawing and their larvae return to ocean after metamorphosis, e.g. lamprey, hagfish. 2. Class-Chondrichthyes

⚫ These are cartilaginous fishes.

• Notochord is well-developed and persists throughout life.

• Mouth is on the ventral side and teeth are modified

2 placoid scales.

⚫ Heart two-chambered, ureotelic animals, sexes are separate, males usually have claspers for copulation, e.g. sharks (Carcharodon and Sphyrna), sting rays (Trygon), etc.

• Some have electric organs (e.g. Torpedo) and others have poison sting (e.g. Trygon). They are poikilothermous (cold blooded, i.e. they lack the capacity to regulate their body temperature) animals. Possess uncovered gills and five pairs of gill slits and tough skin containing minute placoid scales.

• Some are predaceous (e.g. sharks).

• Sexes are separate and fertilisation is internal. Many are viviparous, e.g. Carcharodon, Sphyrna, etc.

3. Class-Osteichthyes

⚫ These are marine as well as freshwater bony fishes.

• Four pairs of filamentous gills, covered by operculum (gill cover) are present. Exoskeleton with cycloid or ctenoid scales.

They contain air bladder that regulates buoyancy. ⚫ Heart is two-chambered and mostly of these are ammonotelic.

• Sexes are separate and development is direct. Mostly they are oviparous.

⚫ Fertilisation external, e.g. Labeo, Catla, Clarias, etc.

4. Class-Amphibia

⚫ These are the first terrestrial organisms and can live in both aquatic and terrestrial habitats. These are poikilothermic, ectothermic or cold-blooded.

Body is divided into head and trunk. Tail may be present in some.

• Heart is three-chambered. These have mesonephric kidneys and mostly are ureotelic.

Alimentary canal, urinary and reproductive tracts open into cloaca.

• Respiration occurs by gills, lungs, lining of buccopharyngeal cavity and moist skin, either, separately or in combination.

• Tympanum represents the ear.

The eyes have eyelids and inner and middle ears represent the ear. Fertilisation is external. These are oviparous and the development is direct, e.g. Salamandra, Rana (Frog).

5. Class-Reptilia

• They show creeping or crawling movements and are mostly terrestrial.

• They are poikilothermic, exothermal or

cold-blooded animals.

⚫ Body covered by dry epidermal and cornified

scutes or scales and their skin lacks glands.

• Kidney is metanephric. Crocodiles are ammonotelic, turtles and alligators are ureotelic and lizards and snakes are uricotelic.

• Bony endoskeleton, well-developed digestive system, respiration through lungs (in turtles through cloaca), heart is three-chambered (except in crocodiles that have four-chambered heart).

• Sexes are separate and fertilisation is internal. They are oviparous and show direct development, e.g. Chameleon, Gavialis.

6. Class-Aves

. These animals are characterised by the presence of feathers that act as insulator and help in flight. Body is streamlined.

• They possess beak and forelimbs that are modified into wings.

They are endothermal and warm-blooded

(homeothermous) animals.

• Bony endoskeleton, feathery exoskeleton metanephric kidneys and have pneumatic bones (with air cavities).

• Digestive tract contains crop and gizzard. Heart is four-chambered and respiration occurs through lungs. These are uricotelic and have a special voice producing organ called syrinx.

⚫ Sexes are separate. Fertilisation is internal. They are oviparous and show direct development, e.g. crow, pigeon.

7. Class-Mammalia

. These are characterised by the presence of milk producing mammary glands and give birth to young ones, i.e. show viviparity. • They have two pairs of limbs and are adapted to fly

(bat) or live in water (whale) or are terrestrial

(horse, camel and human).

°They are homeothermal and the skin is covered with hairs and have external ears or pinnae.

Body is divisible into head, neck, trunk and tail. The heart is four-chambered and respiration occurs through

lungs.

• Sexes are separate, fertilisation is internal and developmnent is direct, e.g. platypus, kangaroo, camel, etc.