Different Categories of Animals

Animal groups

Methods of grouping animals

There are several ways of grouping animals. In all these methods the basic Taxon remains without any change. However the taxa are rearranged in different groups. All these groupings are mostly provided for the convenience in identifying similar taxa.

I. One of the earliest method of grouping the animals could be dividing the Animal kingdom into two assemblages called Invertebrata and Vertebrata This scheme was provided initially by Aristotle. This scheme does not have a place for the Prochordates.

II. Animals can also be grouped as single celled and multicellular. The single celled organisms are called the Protozoans. The multicellular could be called the Metazoans. In this arrangement among the metazoans the unique nature of the sponges in not having a tissue grade of body constuction is not mentioned.

III. In yet another method the animals are grouped under following three assemblages.

1. Protozoa - single celled animals
2. Parazoa - Multicellular without tissue grade (sponges).
3. Eumetazoa - Multicellular with tissue grade.

Eumetazoa is a large group including most of the multicellular animals. Hence it is subdivided further into two groups.
1. Diploblastic animals - having ectoderm and entoderm as two layers in the body wall. Ex : Coelenterata.
2. Triploblastic animals - having ectoderm, mesoderm and endoderm as three layers in the body wall

The Triploblastic animals are further divided into three groups based on the presence or absence of an embryonic body cavity called coelom
1. Acoelomata - no coelom Ex : Platyhelminthes
2. Pseudocoelomata - with a false coelom Ex : Nematoda
3. Coelomata - with a true coelom

IV. In a recent system, the entire living world is subdivided into 5 kingdoms. This system is much more broader including algae, fungi, and plants. It is known as the Five kingdom concept

1. Kingdom : Monera - It includes all bacteria and the cyanobacteria. A circular DNA occurs in the cytoplasm. The cell wall is a rigid structure
a) Phylum :Cyanobacteria
b) Phylum : Bacteria

2. Kingdom : Protoctista or Protista - It includes single celled eukaryotes. It has two subkingdoms, namely Protozoa and Algae.
3. Kingdom : Fungi
4. Kingdom : Plantae (green plants)
5. Kingdom : Animalia : multicellular, eukaryotic animals

Major phyla

Phylum : Protozoa   This phylum includes a great diversity of small, microscopic organisms. These are single celled eukaryotes. Their locomotion happens using pseudopodia, cilia or flagella.

The nutrition is either autotrophic or heterotrophic. They reproduce either asexually or by sexual methods. Ex : Amoeba, Paramoecium, Plasmodium.

Phylum : Porifera.  These are multicellular, aquatic organisms. They have a cellular grade of construction without the occurrence of tissues. The sponges belonging to this phylum are characterised by the presence of a canal system in their body. The body wall contains spicules. They can reproduce both by asexual and sexual methods.
Ex : Sponges.

All coelenterates are aquatic animals. They are mostly marine. The body is radially symmetrical. The body wall is of two layers of cells. The outer layer is called the ectoderm. The inner layer, entoderm is seperated from the ectoderm by a non-cellular mesogloea. The mesogloea is a jelly-like substance. Due to the presence of two layers in the body wall, these are said to be diploblastic animals. Many coelenterates exhibit polymorphism. In this phylum, organisms exist in two different body forms namely, a polyp, and a medusa.The ectoderm contains stinging cells called nematocysts (cnidoblasts). These cells when triggered can explosively penetrate prey and inject poison.

The layers in the body wall contain several cells and tissues such as muscle cells epithelial tissues, gland-cells and sensory cells.

They reproduce both asexually and sexually. They are divided into three classes, namely Hydrozoa, Scyphozoa and Anthozoa. In Hydrozoa, the animal has a dominant polyp body form and a reduced medusa stage. (e.g) Hydra, Obelia. In Scyphozoa the medusa form is permanent. This group includes jelly fishes such as Aurelia. They swim in the surface waters. They have a bell shaped medusa stage. The Anthozoans mostly remain as polyps. Their body cavity is divided by large radial partitions called mesenteries. (eg) sea-anemone and corals. All animals of subsequent phyla show the following general characters.

1. All of them have three layers in the body wall. They are named as outer ectoderm, middle mesoderm, and inner endoderm. Thus they are called as Triploblastic animals.

2. The body is bilaterally symmetrical

Phylum: Platyhelminthes :- This phylum includes flatworms. These are acoelomates, without a body cavity called coelom. The alimentary canal is either absent or very simple. Excretion and osmoregulation occur through flame cells.These worms are mostly hermophrodites, having both male and female reproductive organs in a single individual. Most of the members are parasites. It is divided into three classes, namely Turbellaria, Trematoda and Cestoda

Class Turbellaria :- These are free living aquatic flatworms. The Planaria of this class shows characteristic regeneration.

Class Trematoda :- These are flukes living as parasites inside a host (endoparasites). A protective cuticle covers the outer surface of the body. Flukes have suckers for attachment to the host tissues. The examples are Fasciola (liver fluke), Schistosoma (blood fluke).

Class Cestoda :- It includes all tape worms. These are internal parasites with a complex life history. The life cycle involves two hosts. Their body characters are adaptations for parasitic life. Mouth and alimentary canal are absent. Food is absorbed through general body surface. The head is called the scolex. It has a ring of hooks and suckers for attachment to the host tissue. The body consists of several segments called Proglottids. (eg) sheep and cattle tape worms.

Phylum : Nematoda :-  These are the popular round worms. The body is narrow and pointed at both the ends. There are no body segments. The body is covered by a thin cuticle. The body cavity is considered as a pseudocoelom. The alimentary canal is a straight tube. They reproduce sexually and the sexes are seperate. There are several free living soil nematodes. Others are parasites. (eg) Ascaris lumbricoides.

In subsequent Phyla the animals show following general characters
1. There is a coelom within the mesoderm. Hence these are called as coelomates.
2. The body consists of a series of compartments. This phenomenon is called as metameric segmentation. They have a circulatory system providing internal transport.

Phylum: Annelida :-  These are worm like animals. The body segments are rings externally. Internally the segments are seperated by septa. Externally the body is protected by a cuticle. Excretion and osmoregulation are acheived by ciliated tubules called nephridia. There is a central nervous system. The brain is formed of ganglia in the head region. The nerve cord is ventral in position. For the first time head formation or cephalization happens. These are bisexual and hermophroditic. The larva is called the trochophore

This phylum includes three Classes, namely Polychaeta, Oligochaeta and Hirudinia. The polychaetes are marine worms. They have a distinct head. There are pairs of lateral projections called parapodia. The examples are Nereis (ragworms), Arenicola (lugworm). Earthworms are included in the Class Oligochaeta.

The Class: Hirudinia includes leeches. These are blood suckers and ectoparasites. They have well developed suckers for attachement at anterior and posterior ends.

Phylum : Arthropoda :- These are the most successful group of animals. They outnumber all other animals in population strength. The body is segmented. It is covered by a hard exoskeleton made of chitin. During growth the exoskeleton is shed (moulting of ecdysis). The legs or paired appendages are jointed. The head region has a pair of prominent compound eyes. Each compound eye is made
up of several photoreceptor sub units called Ommatidia.

They have an open circulatory system without vessels. The body cavity is filled with a fluid called haemolymph. Such body cavity is known as haemocoel. These are unisexual, exhibiting sexual dimorphism. The young forms produced are invariably called the larvae. The larvae undergo metamorphosis and develop into adults. This Phylum comprises five Classes, Class Onychophora: It includes small worm like Peripatus. Peripatus shows Annelidan and Arthropoda characters. Hence this may be considered as a connecting link between the two groups.

Class Crustacea :- The examples for this class are prawns, crabs and lobsters. The dorsal body surface is covered by a sheild like carapace

Class Myriapoda :- It includes centepedes and millipedes. These organisms have a distinct head and  simple eyes. The centepedes have a pair of poison claws. The body consists of numerous segments, bearing pairs of legs.

Class Insecta :- It comprises the common insects. The body is divided into head thorax and abdomen.In several insects, the adults have two pairs of wings on the thorax. Respiration happens through the tracheal system.

Class Arachnida :- It includes scorpions, spiders, ticks and mites. The body is divided into cephalothorax and abdomen. There are four pairs of legs attached to the cephalothorax.

Phylum Mollusca :- It is a very successful and diverse group of animals. Considered to be the second largest group of animals with regard to species number. These are soft bodied animals without segmentation. The body is divided into head, muscular foot and visceral mass. The body is covered by a mantle and a shell. Respiration happens through gills (ctinidia) in the mantle cavity. The most common larva is a trochophore larva.

There are seven classes of which three are more prominent

Class Pelecypoda or Bivalvia :- These are aquatic molluscs having bivalves. They burrow in mud and sand. The body is laterally compressed. (eg) mussels, clams, oysters

Class Gastropoda :- These are either aquatic or terrestrial molluscs. They posses a spiral shell. The foot is large and flat. They have well developed head with tentacles and eyes. (eg) snails, slugs, and limpets.

Class Cephalopoda :- These are mostly marine. They are adapted for swimming. The foot is modified into eight to ten long tentacles in the head region. The shell is either internal or absent. (eg) Octopus, Loligo, Sepia.

Phylum Echinodermata :- These are marine organisms. While the adults are radially symmetrical the larvae remain bilaterally symmetrical. The mouth is on the lower surface. They have a water vascular system with tube feet. eg. star fishes, brittle stars, sea urchins and sea-cucumbers.

Phylum Chordata This phylum derives its name from one of the common characteristics of this group namely the notochord (Gr. noton, back + L. chorda, cord). The animals belonging to all other phyla of the Animal Kingdom are often termed ‘the non -chordates’ or ‘the invertebrates’ since they have neither notochord nor backbone in their body. The backboned animals (vertebrates), together with a few closely related animals which do not possess a backbone, are included in this phylum. Most of the living chordates are familiar vertebrate animals. The chordates are of primary interest because human beings are members of this group.

Diversity of Chordates  The chordates exhibit an astonishing diversity in form, physiology and habits. The number of chordate species is limited. About 49,000 species are on record which are only half of the living species of molluscs and less than one tenth of arthropods. Despite their modest number of species, the chordates make remarkable contribution to the bio-mass of the earth. Nearly all of them are medium to large in size. The vertebrates in particular are considerably larger and many of them are among the largest of living animals. The gigantic blue whale which is 35 meters long and 120 tons in weight is the biggest known animal. The smallest vertebrate , philippine goby is a fish, measuring only 10 mm in length. The chordates are able to occupy various kinds of habitats. They have adapted themselves to more modes of existence than any other group. They are found in the sea, in freshwater, in the air and on all parts of land from the poles to the equator.

General Characters :  The three distinctive characteristics of the chordates are the presence of notochord, dorsal tubular nerve cord and pharyngeal gill slits.

1. Notochord :  During the embryonic development of a chordate there appears a supporting rod called the notochord. It lies dorsal to the alimentary canal and ventral to the nerve cord. In some chordates this structure persists throughout life. In others it is partially or completely replaced by a ‘backbone’. It is made up of separate bony elements or vertebrae. Structurally it is composed of large number of specialized vacuolated cells. It is surrounded by fibrous and elastic sheath. The stiffness of the notochord is due to the turgidity of fluid-filled cells and surrounding connective tissue sheath.

2. Dorsal tubular nerve cord The nerve cord lies just above the notochord and remains entirely outside the coelom. It is a tubular structure having a small hollow canal running from one end to the other. The dorsal hollow nerve cord persits throughout the adult life of almost all chordates.

3. Gill slits or Pharyngeal clefts  These are paired lateral clefts leading from the pharynx to the exterior. They are present throughout life in fishes and a few tailed amphibians. In amphibians, like frogs and toads it is found only in the larval stages. In higher vertebrates (reptiles, birds and mammals) they are embryonic and non-functional.

4. Ventral heart The heart is chambered. It is located ventral to the alimentary canal.

5. Closed blood vascular system In chordates, the blood passes through a continuous system of tubes namely arteries, capillaries and veins.

6. Hepatic portal system In chordates, the food laden blood from the digestive tract passes through the capillary net work in the liver, before reaching the heart. Thus the veins originating from the digestive tract as capillaries and ending in the liver again as capillaries constitute the hepatic portal system.

Classification.
The Phylum Chordata is classified into four sub phyla:
Sub phylum 1. Hemichordata,
Sub phylum 2. Cephalochordata
Sub phylum 3. Urochordata
Sub phylum 4. Vertebrata.

First three sub phyla are collectively known as Protochordates. Since the members of these sub phyla do not have a cranium or skull they are also referred to as Acrania.

Protochordata (Acrania)  The protochoradates are considered as the fore runners of vertebrata
The classification of the protochordates is based on the nature of the notochord.

Sub phylum : Hemichordata. These are exclusively marine organisms. They are solitary or colonial
forms. They mostly remain as tubiculous forms. The body is soft, vermiform, unsegmented,bilaterally symmetrical and triploblastic. The body is divisible into three distinct regions namely proboscis, collar and trunk. The body wall is composed of single layer of epidermal cells. The dermis is absent. They have no endoskeleton. A projection from pharynx, projecting inside the proboscis may be considered as notochord. They have a spacious coelom lined by coelomic epithelium. The alimentary canal is a straight tube running between mouth and anus. They are ciliary feeders. Sexes are separate.
Examples : Balanoglossus, Saccoglossus

Sub phylum : Cephalochordata. Cephalochordates are small fish like marine chordates. The persistent notochord extends forward beyond the brain. Hence these are called cephalochordates. The epidermis is single layered. Paired fins are absent. Muscles, nephridia and gonads are segmentally arranged. The pharynx is large with numerous gills. It is a filter feeder.

Sub phylum : Urochordata This taxon constitutes a unique group of animals exhibiting diversity in
form and habit. In Urochordata the notochord is confined to the tail region of the larva. The adults are mostly degenerate, sessile forms. The body is enveloped by a tunic or test. The free end of the body bears two openings, the mouth and the atriopore. The proximal part of the alimentary canal is greatly enlarged to form a spacious pharynx. They are hermaphroditic animals. The development occurs through free swimming tadpole like larva.
Example : Ascidia, Doliolum, Salpa.

Sub phylum : Vertebrata (Craniata) This group is characterized by the presence of brain case or cranium and a vertebral column which forms the chief skeletal axis of the body. The notochord is an embryonic structure. It is replaced in the adult stage by a cartilaginous or bony vertebral column. The body is covered with an integument having an outer epidermis and an inner dermis. The skin has many modifications such as glands, scales, feathers, claws horns and hairs

The digestive system is ventral to the vertebral column. It is provided with a large liver and pancreas. The circulatory system consists of the ventral, chamberd heart. The circulatory system is of a closed type with arteries, veins and capillaries. The blood plasma contains red and white blood corpuscles. Gill slits are limited in number (usually 5 pairs). There are two pairs of appendages. The anterior part of the nerve cord becomes differentiated into brain and spinal cord. The special organs of sense like the nose, eyes and ears are closely connected with the brain. Urinary and genital systems are closely connected to form an urinogenital system
The sub phylum vertebrata may be classified into two groups
(i) Pisces and
(ii). Tetrapoda

Class : Pisces Fishes are poikilothermic, aquatic vertebrates with jaws. The body is streamlined. It is differentiated into head, trunk and tail. Between head and trunk, the neck is absent. Locomotion is effected by paired and median fins.

The body has a covering of scales. They are of various types like placoid, cycloid, ctenoid and ganoid scales. The body muscles are arranged into segments called myotomes. The Alimentary canal consists of a definite stomach and pancreas and terminates into cloaca or anus. Respiration is performed by gills. Gill slits are 5-7 pairs. They may be naked or covered by an operculum. The heart is two chambered (an auricle and a ventricle).

Sinus venosus and renal portal system are present. The red blood corpuscles are nucleated. The functional kidney of the adult is of mesonephric type. The external nostrils do not communicate with the buccal cavity. Lateral line sense organs are well developed. Sexes are separate. Fertilization is either internal or external . Examples: Shark, Catla.

Tetrapoda

The vertebrates with two pairs of limbs adapted for locomotion on land are known as tetrapods. The limbs are of pentadactyl type. The tetrapods are identified by a cornified outer layer of skin and nasal passages communicating with mouth cavity and lungs. The super class Tetrapoda is divided into four classes namely. Amphibia, Reptilia, Aves and Mammalia

Class : Amphibia The living representatives of this class include frogs, toads, newts, salamanders and limbless caecilians.

The transition from aquatic to terrestrial living is clearly indicated in the class Amphibia. These were the first vertebrates to live on land. Amphibians are not completely land adapted. They hover between aquatic and land environments. This double life is expressed in their name, amphibia. It is because of, these reasons ‘the amphibians are considered, a defeated group’. The body forms vary greatly from an elongated trunk with distinct head, neck and tail to a compact, depressed body with fused head and trunk and no intervening neck. The forelimbs of frogs and toads are smaller than hind limbs. In frogs, hindlimbs have webbed feet. The surface of the skin is smooth and slimy. The slimy nature is due to the presence of mucous secreting glands. Scales are practically absent. The mouth is usually large with small teeth in upper or both jaws. The external nostrils communicate into the anterior part of the mouth cavity. Respiration is effected by gills, lungs, skin and pharyngeal region. The heart is three chambered with two auricles and a single ventricle. The skeleton is mostly bony, with varying number of vertebrae; exoskeleton is absent. Sexes are separate. Fertilization is either external or internal. The tadpole Metamorphoses into adult.
Examples : Frog, Toad, Salamander, Caecilian

Amniota

The tetrapods like reptiles, birds and mammals are referred to as amniotes. The amniotes have certain membranes associated with embryos inside the egg. It is an adaptation in terrestrial forms during development. These membranes are the amnion, chorion and allantois.

Class : Reptilia Reptiles are represented by lizards, snakes, turtles, tortoises, alligators, crocodiles and the tuatara lizard, Sphenodon punctatum.

The body is variable in shape. It is covered with an exoskeleton of horny imbricate epidermal scales. Skin glands are practically absent. The limbs are of pentadactyl type adapted for climbing, running and paddling. The endoskeleton is well ossified. Respiration is by lungs. The heart is three chambered (In crocodiles it is four chambered). The functional kidney of the adult are metanephros. The Sexes are separate. Fertilization is internal. The eggs are covered with leathery shells. Reptiles have developed some form of copulatory organ to transfer the sperms into the cloaca of the female.
Example : Garden lizard, Cobra, Monitor lizard, Crocodile, Turtle.

Class : Aves Birds are one of the most intersting and widely known group of animals. There are more than 8600 species of birds distributed all over the world. Birds as a group exhibit a characteristic uniformity in structure. Aves are warm blooded vertebrates with an exoskeleton of feathers forming a non-conducting covering to keep the body warm. The feet are covered with scales. The forelimbs are modified as wings and provided with feathers for flight. The hindlimbs are attached far forwards to balance the weight of the body. The bones are spongy, containing air-cavities rendering the body light. There is a fusion of bones and this is especially seen in the vertebral column. Only three digits are present in the forelimbs. In the hindlimbs there are four toes with the first directed backwards. A horny beak is present.

The alimentary canal ends in a cloaca. Inside the body air sacs are present and some of them communicate with air cavities in the bones. The heart is four chambered. The red blood corpuscles are oval and nucleated. The kidneys are three lobed. The ureters open into the cloaca. Urine is semisolid and contains uric acid. The nervous system is well developed. Eyes are usually powerful and a specialized structure called pecten is present inside the eye ball to help in accomodation. Sexes are separate, Fertilization is internal. Eggs are provided with large amount of yolk. The egg is covered by a hard calcareous shell. In spite of several advanced features the birds have certain reptilian characters. Hence they are known as “glorified reptiles”.
Examples : Pigeon, parrot, crow, sparrow, peakcock, ostrich, penguin

Class : Mammalia The term “mammalia” was given by Linnaeus (1758) to that group of animals which are nourished by milk from the breasts of the mother. They are a successful group, for they adapt themselves readily to new situations and to new food habits. The body is generally covered with epidermal hairs. The integument is provided with sweat, sebaceous and scent glands. The mammary glands are modified integumentary glands. The external ear or the pinna is present in most of the mammals. A muscular diaphragm is present in between thoracic and abdominal cavities. It helps in respiration. The red blood corpuscles are non-nucleated, biconcave and usually circular in form. The heart is four chambered. Only the left aortic arch is present. In brain cerebral hemispheres are very large and highly convoluted.

Corpus callosum, a transverse band of nerve fibres connecting the two cerebral hemispheres, is present. Dentition is thecodont, heterodont and diphyodont. Cloaca is absent. Testes lie outside the body cavity, enclosed in scrotal sacs. Eggs are small with little or no yolk. Fertilization is always internal. Mammals are Viviparous ie., they give birth to alive young ones. Placenta is usually present

The class Mammalia is subdivided into three subclasses namely Monotremata, Marsupialia and Placentalia

1. Sub class : Monotremata or Prototheria

These are primitive egg laying mammals Example : Spiny ant-eater, duck billed platypus.

2. Sub class : Marsupialia or Metatheria These are popularly called as marsupials or pouched mammals. The young ones are born in an immature stage and migrate into the pouch on the mother’s body. Further development is completed in the pouch or marsupium.
Example : Kangaroo

3. Sub class : Placentalia or Eutheria In this group eggs develop within the uterus. The developing embryo receives nutrition through maternal blood circulation via the placenta.
Example : Elephant, tiger, lion, man, monkey, dog, cat , rat, bat.

Order Primates : It is an order coming under the subclass Eutheria. This order is of interest because it includes man, besides lemurs, tarsiers, monkeys and apes. They inhabit chiefly the warmer parts of the world. This group stands first in the animal kingdom in brain development. However, most of them are unspecialized and tree dwelling (arboreal). Primates are omnivorous in habit. The body is covered with hairs except palm, sole and parts of face. The neck is mobile. The forelimbs are shorter than the hindlimb. The limbs have five digits and all the digits end in flat nail. The pollex or thumb or first toe are smaller than other digits and are opposable (except the hallux of man). The brain is highly developed. The cerebral hemispheres are much convoluted and cover the cerebellum. The eyes are directed forward and the vision is binocular and stereoscopic. Mammae are two and thoracic in position.

To know

	Invertebrates	Scientific Names
1	Earthworm	Lampito mauriti
2	Cockroach	Periplaneta americana
3	House fly	Musca nebula
4	Locust	Schistocera gregaria
5	Bed bug	Cimex hemipterus
6	Leaf insect	Phyllium sps
7	Stick insect	Carausius sps
8	Water-scorpion	Nepa sps
9	Butterfly	Pieris sps
10	Rat flea	Xenopsylla cheopis
11	Scorpion	Palamnaeus swammerdami
12	King crab	Limulus sps
13	Spider	Aranea sps
14	Apple snail	Pila globosa
15	Freshwater mussel	Lamellidens marginalis
16	Star fish	Asterias rubens

Vertebrates
1	Angel fish	Pterophyllum scalare
2	Guppy	Poecilia reticulata
3	Frog	Rana hexadactyla
4	Garden lizard	Calotoes versicolor
5	Cobra	Naja naja
6	Peacock	Pavo cristatus
7	Crow	Corvus splendens
8	Sparrow	Passer domesticus
9	Parrot	Psittacula Krameri
10	Rat	Rattus rattus
11	Dog	Canis familiaris
12	Cat	Felis domesticus
13	Tiger	Panthera tigris
14	Lion	Panthera leo
15	Elephant	Elephas maximus
16	Man	Homo sapiens
17	Monkey	Macaca radiata
18	Mongoose	Herpestes edwardsii
19	Bear	Ursus arctos
20	Fruit bat	Cynopterus sphinx
21	Donkey	Equus hemionus
22	Rhinoceros	Rhinoceros unicornis
23	Spotted deer	Axis axis
24	Man	Homo sapiens