ANIMAL KINGDOM

Classification of Animals

We just discussed that we are surrounded by a vast range of animals. Now we cannot study these animals haphazardly without any classification. So for a systematic study of animals, they are divided into various minor and major groups on the basis of similarities and differences and each group is given a particular name depending upon its characteristics. There are various levels of classification, like kingdom, phylum, class, genus etc. which in the decreasing order of complexity can be shown as:

Kingdom ® Phylum ® Class ® Order ® Family ® Genus ® Species

Basically the Kingdom Animalia is divided into following eleven phyla:

(1)   Porifera

(2)   Coelenterata (Cnidaria)

(3)   Ctenophora

(4)   Platyhelminthes

(5)   Aschelminthes

(6)   Annelida

(7)   Arthropoda

(8)   Mollusca

(9)   Echinodermata

(10) Hemichordata

(11) Chordata

Noteworthy Points: Do You Know?

1.      Hemichordata, Urochordata, Cephalochordata which are Acraniates are considered as primitive and often referred to as protochordates or non-vertebrate chordates.

2.      Ascidia: Its intestine has typhlosole. Typhlosole is a fold of epithelium in the intestine. It prevents the rapid flow of food in the intestine and increases the absorptive surface.

3.      Doliolum: Doliolum is a cosmopolitan, marine free swimming and pelagic, urochordate inhabiting all seas.

4.      Vertebra: Vertebra is a Latin word whose meaning is joint.

5.      Pyrosoma a Urochordate, lives in colony, is a phosphorescent form which is due to the symbiotic association of its egg with luminescent bacteria.

6.      The Kingdom Animalia is divided into phyla each one of which includes the animals with common fundamental characteristics.

7.      Porifera includes the primitive multicellular animals which exhibit cellular level of organisation and have characteristic flagellated cells called choanocytes. Its members commonly called sponges have a peculiar canal system in their body which facilitates the free water entry and exit.

8.      The coelenterates, also called cnidarians, have tentacles and bear characteristic cnidoblasts. These show metagenesis with two basic body forms, polyp and medusa. They are mostly aquatic; sessile or free-floating.

9.      The ctenophores are marine animals with comb plates as locomotory structures. These show bioluminescence and exhibit tissue level of organisation.

10.  The platyhelminthes have flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers and hooks. Specialised flame cells help in the excretion and osmoregulation.

11.  Aschelminthes are pseudocoelomates and include parasitic as well as non-parasitic roundworms. The muscular pharynx helps in sucking the food. Male and female are distinct.

12.  Annelids are metamerically segmented animals with a true coelom. Nephridia help in the excretion and osmoregulation. They may be aquatic or terrestrial, free-living and sometimes parasitic.

13.  Arthropoda is the largest phylum of animal kingdom. Arthropods are the most abundant group of animals characterised by the jointed appendages. The body is covered with chitinous exoskeleton. Circulatory system is of open type. Excretion takes place through malpighian tubules. They may be terrestrial or aquatic or aerial.

14.  The molluscs have a soft body surrounded by an external calcareous shell. Body 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. Mollusca are the second largest phylum of animal kingdom.

15.  The echinoderms possess a spiny skin and their most distinctive feature is the presence of water vascular system which helps in the locomotion, capture and transport of food and respiration.

16.  The hemichordates are a small group of worm-like marine animals. They have a cylindrical body with proboscis, collar and trunk. Sexes are separate and circulatory system is of open type.

17.  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, paired pharyngeal gill slits and post anal tail.

18.  Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata). Division Agnatha is represented by the class. Cyclostomata. They are the most primitive chordates and are ectoparasites on fishes.

19.  Division Gnathostomata has two super classes. Pisces and Tetrapoda. Pisces include classes chondrichthyes and osteichthyes which bear fins for locomotion and are grouped under pisces. The chondrichthyes are fishes with cartilaginous endoskeleton and are marine, while osteichthyes have bony endoskeleton and or both marine and fresh water dwellers.

20.  Tetrapoda include classes Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thus grouped under Tetrapoda.

21.  The amphibians have adapted to live both on land and water.

22.  Reptiles are characterised by the presence of dry and cornified skin. Limbs are absent in snakes.

23.  Fishes, amphibians and reptiles are poikilothermous (cold-blooded).

24.  Aves and mammals 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.

25.  The unique features of mammals are the presence of mammary glands and hairs on the skin. They commonly exhibit viviparity.

Sub-phylum - 3. Vertebrata or Craniata

Division-1: Agnatha (The Jawless Vertebrates)

Class 1. Cyclostomata

Division-2: Gnathostomata (The Jawed Vertebrates)

Superclass-1: Pisces

Class-2: Osteichthyes

Superclass-2: Tetrapoda

1.      These possess two pairs of limbs.

2.      Skin is adapted to withstand exposure to air.

3.      They have lungs for aerial respiration.

4.      Their sensory organs are adapted for reception in air, such as vision, hearing, smelling etc.