Animals are divided into two main groups; Nonchordates and Chordates

Animals are divided into two main groups; Nonchordates and Chordates.

A. Nonchordates:

Nonchordates include those animals which are devoid of a backbone or vertebral column. They constitute about 95% of the known animals and can be divided into the following groups:

1. Protozoa:

They are small, acellular organisms. Their bodies are not differentiated into cells. A protozoon is made up of a mass of protoplasm which carries all the vital activities of life such as locomotion, feeding, digestion, egestion, respiration, excretion and reproduction.

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Although it is not divided into cells, it has specialization and division of labour within its cytoplasmic mass. Protozoa on the basis of means of locomotion is divided into four classes;

(i) Sarcodina with pseudopodia (Amoeba),

(ii) Mastigophora with flagella (Euglena and Trypanosoma)


(iii) Ciliata with cilia (Paramoecium and Vorticella) and

(iv) Sporozoa with no locomotory organ.

2. Porifera:

Porifera includes the sponges which are the most primitive of the multicellular animals. They remain fixed to some solid rock and are incapable of any movement. Their bodies are perforated by pores and canals, but there are no organs such as mouth and nervous system. Digestion takes place within cells, e.g. Leucosolenia, Sycon, etc.


3. Coelenterata:

Coelenterates are radially symmetrical animals with a two-layered body, epidermis and gastrodermis. The space between these two layers is filled with ajelly-like substance known as mesogloea. All the functions of the body are performed by tissues and never by organs.

The mouth is used both for ingestion and egestion and there is no coelom. The nervous system is a primitive nerve net. There is an oval, ciliated planula larva during development, e.g., Hydra, Jelly-fish, Sea-anemone and Corals.

4. Platyhelmintlies (Flatworms):

The bodies of platyhelminthes are compressed dorsoventrally and show bilateral symmetry. The mesoderm forms a type of connective tissue called parenchyma which fills the body space between ectoderm and endoderm so that there is no coelom or haemocoele and hence they are called acoelomate animals. The excretory system consists of flame-cells. Blood vascular system and respiratory system are absent.

There is no anus and in some, even mouth and alimentary canal are also absent. Reproductive organs are highly developed. Platyhelminthes is divided into three classes; (i) Turbellaria includes ciliated free-living flatworms (Dugesia) (ii) Tremaloda are non-ciliated parasitic flatworms or flukes and (iii) Cestoda are all endoparasitic flatworms or tapeworms.

5. Aschelininthes (Nematoda):

The body of nematoda is bilaterally symmetrical with a smooth surface. There is no well-formed head and cilia; the excretory system has no flame cells but a single external pore. There is a complete digestive tract which is straight and has a mouth and anus with a long pharynx having triradiate lumen. The circulatory and respiratory systems are absent.

Nematodes are dioecious with distinct sexual dimorphism, the male organs open into a cloaca while the female organs open by a gonopore, e.g., Ascaris, Enterobius (Pin worm), Ancylostoma (Hook worm), Dracunculus (Guinea worm), Wuchereria (Filaria worm) and Trichur (Whip worm).

6. Annelida:

Annelida are triploblastic; bilaterally symmetrical, segmented metazoa. The body is divided for the first time in the animal series into segments (metameres) arranged in a linear series. The coelom contains a fluid which acts as hydraulic skeleton.

The nervous system has a pair of pre-oral ganglia or brain and the circulatory system is of closed type. Digestion is entirely extracellular and the digestive tract is well differentiated into regions. Organs of excretion are ectodermal nephridia. The larval form is trochosphere, e.g., Pheretima (Earthworm), Hirudinaria (Leech) and Myzostoma.

7. Arthropoda:

Arthropoda is the largest group of the animal kingdom. They are bilaterally symmetrical organisms in which the body is divided into head, thorax and abdomen and is covered with a thick chitinous cuticle. Body segments usually bear paired lateral and joint appendages.

Digestive tract is complete; moutl and anus lie at opposite ends of the body. The circulatory system is of open type with dorsal heart and arteries but without capillarjes. Respiration is through general body surface or by tracheae or booklungs.

True nephridia for excretion are absent. Excretion occurs through malpighian tubules or green or coxal glands. Sexes are generally separate and fertilization is internal. Development is usually indirect through larval stages.

Parental care is well-marked in many arthropods, e.g., Palaemon (Prawn), Palameneus (Scorpion), Periplaneta (Cockroach), Locust (Grasshopper), Culex and Anopheles (Mosquitoes), Musca (Housefly) and Apis (Honey bee)

8. Mollusca:

Molluscs are aquatic animals with soft, unsegmented, bilaterally symmetrical bodies. The body consists of head. Foot, mantle and visceral mass and is protected by a calcareous shell. The head is distinct bearing the mouth and is provided with eyes, tentacles and other sense organs.

Digestive tract is simple with an anterior mouth and posterior anus. Pharynx contains a rasping organ, the radula. Respiratory organs consist of numerous gills or ctenidia provided with osphroadium at the base. The lung is developed in terrestrial form.

The excretory system consists of a pair of metanephridia which communicates with the exterior by the nephridiopore. The nervous system contains paired cerebral, pleural, pedal and visceral ganglia joined by nerves.

Sexes are usually separated (dioecious) and fertilization may be external or internal. Development is either director with metamorphosis through trochophore stage called veliger larva, e.g., Pila (Snail), Unio (Water- mussel), Chiton and Mytilus.

9. Echinodermata:

Echinodermata forms a well defined and successful group of marine animals. The body is made up of 10 principal divisions which radiate from a main axis; 5 radii and 5 inter radio. The surface having a mouth is called oral or ambulacral and the opposite surface is known as aboral or adambulacral. Many echinoderms possess a faculty of self-mutilation or autotomy by which they can break off their arms when molested. They also possess power of regeneration.

10. Ctenophora (Comb-jellies):

The ctenophores are marine animals found abundantly in warmer seas. They are of planktonic habit floating in the surface water. Both the sexes are present in the same-individual (hermaphrodite). The gonads develop in the meridional canals and ova and sperms are discharged into the canals and finally escaoe by mouth. Fertilization is external, e.g., Hormiphora.

11. Rotifera:

The rotifers (wheel animalcules) are free living aschelminthes with anterior end formed into a ciliary apparatus known as corona which consists of pharynx, provided with jaws and typical flame-bulb protonephridia. Rotifers are exclusively dioecious and show sexual dimorphism; males are smaller than-females, e.g., Brachionus.

12. Hemichordata:

Hemichordata are tongue worms which were included into chordata for a long time but now they are placed as independent phylum of invertebrate. The body and coelom are divisible into three unequal regions viz, proboscis, collar and trunk. Nervous system is a network of nerve cells and fibres on the surface of the body.

They have no bony tissues but a buccal diverticulum is present in the preoral region which is often regarded as notochord. Development is direct in some, while in others, there is a tornaria larva, e.g., Balanoglossus and Saccoglossus.

13. Urochordata:

Urochordata includes a peculiar group of marine animals called sea squirts or ascidians. In most of them, chordate characters are lost in adults but larval forms clearly show these characters such as pharyngeal gill clefts, a dorsal tubular nervous system and a notochord which is confined only to the tail hence the name urochordata. They have no metameric segmentation and coelom is absent. They all are hermaphrodite, e.g., Herdamania, Molgula, Ascidia and Salpa.

B. Chordata:

It includes all the higher animals including man.

These animals at some time in their life history, exhibit the following three primary diagnostic characters:

1. Notochord:

A notochord is an elastic, solid, skeletal rod lying below the nerve cord and above the alimentary canal. It serves as a primitive internal skeleton and acts as a rigid axis.

2. Dorsal Tubular Nerve Cord:

There is a dorsal, hollow, fluid filled nerve cord formed by ectoderm. It lies above the notochord and outside the coelom.

3. Gill-Clefts:

Gill-clefts are paired openings leading from the pharynx to the exterior. The gill-clefts appear during the development of every chordate.

Higher chordates also possess some other characters such as ventral heart, hepatic portal system, R.B.C. and post anal tail. Vertebrate

Vertebrate constitutes a sub-phylum of chordata. Animals belonging to this group have long vertebral column. They possess two pairs of limb, a chambered heart and paired organs of smell, sight and hearing. It is divided into the following groups:

1. Cyclostomata:

They aremarineanimalshaving round bodies with suctorial mouth they are the only living vertebrates without jaws. Endoskeleton is cartilaginous with no bones and the notochord persists throughout life.

Heart is enclosed in cartilage derived from visceral arch. They have no spleen. They attach themselves to the bodies of host by sucker like mouth and eat away their flesh by toothed tongue, e.g., Lampreys and Hagfishes.

2. Agnatha:

They are the earliest known vertebrates characterized by the absence of jaws. Teeth, paired appendages and exoskeleton are absent. Vertebral column consists of a persistent notochord with a fibrous neural tube. They possess a large number of gill-slits which may range between 7 to 14 pairs. They have long kidneys with long archinephric ducts. Larval stage is microphagus, e.g., Petromyzon, Myxine and Cephalaspis.

3. Pisces (Fishes):

They are cold-blooded vertebrates that breathe by means of gills and are aquatic. They usually have streamlined body with paired and unpaired fins supported by spiny fin rays. Dorsal, anal and caudal fins are unpaired while the pectoral and pelvic are paired. Olfactory, photic, auditory and lateral line system (receptors) are well developed.

Sexes are always separate. Inside the body, there is an air bladder which makes it light and enables it to swim. The heart is two chambered; one auricle and one ventricle, e.g., Scoliodon (Dog­fish), Horpodon (Bombay duck), Exocoetus (Flying-fish), Anabas (Climbing perch), Torpedo (Electric ray).

4. Amphibia:

They are cold blooded vertebrates. Skin is smooth or rough and rich in glands which keep it moist. R.B.Cs is biconvex, oval and nucleated. Heart is three-chambered; two auricles and one ventricle. Kidneys are provided with persistent nephrostome. Vagus is the last cranial nerve. They respire by gills, lungs, skin or buccal cavity.

Fertilization is either external or internal and mostly oviparous. Development is always indirect as larva is formed during life cycle which is generally known as tadpole larva. Amphibians live partly in water and partly on land, e.g., Hyla (Tree frog), Rhacophorus (Flying frog), Alytes (Mid-wife toad), Salamandra, Ichthyophis (Blind worm).

5. Reptilia:

These are cold blooded (poi kilo- thermic terrestrial or aquatic animals with dry skin which does not contain any glands) vertebrates. Respiration occurs by lungs. Heart is four chambered; one sinus venosus, two auricles and one ventricle. R.B.Cs is nucleated. Kidney is provided with separate ureter without nephrostomes.

There are 12 pairs of cranial nerves. Embryonic membranes (amnion and allantois) are present. Reptiles are the first true land vertebrates because they have evolved an egg which is laid and develops on land.

Fertilization is internal because male has a single or double penis. They mostly flourished in the Mesozoic era (the era of Dinosaurs), e.g., Chelone (Green turtle), Trionyx (Tortoise), Sphenodon (Tautara), Hemidactylus (Wall- lizard), Draco (Flying lizard), Bungarus(Krait), Naja (Cobra), Crotalus (Rattle snake), Crocodilus (Marsh crocodile).

6. Aves (Birds):

Birds are warm-blooded vertebrates with an exoskeleton of feathers. The forelimbs are modified into wings, each bearing three clawless digits and are provided with feathers for flight.

The hind limbs are adapted for walking, perching, swimming and bear four toes. There is no skin gland except the oil gland (coccygeal) on the tail. The bones are spongy, light in weight containing air cavities which help in flight.

The posterior caudal vertebrae are usually fused to form a pygostyle. Lungs are spongy and non-distensible. The voice is produced by the syrinx situated near the trachea. The heart is four chambered. R.B.Cs are oval, nucleated and biconvex. Kidneys are metanephric, three lobed and ureters open into the cloaca. Sexes are separate and sexual dimorphism is well-marked.

Only left ovary is present and the right one is more or less completely atrophied. Fertilization is internal, oviparous, large ovum containing much yolk. The embryo has an amnion, an allantois and a large yolk sac. Parental care is highly developed, e.g., Columbal (Pigeon), Struthio (Ostrich), Apteryx (Kiwi), Aptenodytes (Penguin), Pavocristatus (Peacock), Eudynamys (Koel), Passer (House-sparrow), Covus (House-crow) and Psittacula (Rose- ringed parakeet or Tota).

7. Mammalia:

Mammals are warm-blooded animals with skin covered with hair. Skin is provided with sudoriporous (sweat) and sebaceous (oil) glands. Mammary gland in female, whose function is to nourish the young, is also present. External ears (pinnae) are well developed.

They generally possess seven cervical vertebrae. The digits in the fore and hind limbs are never more than five. Heart is four chambered; two auricles and two ventricles.

R.B.C. is non-nucleated except in camel. Brain contains four obtic lobes. Kidney is Meta nephros. Penis is always present and hence fertilization is internal.

They are mostly viviparous and young ones develop in the uterus and are born’ alive after some time, e.g., Macropus (Kangaroo), Erinaceus (Hedge-hog), Macaca (Rhesus monkey), Pongo (Orangutan), Anthropithecus (Chimpanzee), Gorilla, Plantanista (Dolphin), Phoca (Seal), Homo sapiens (Man).

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