Classification Of Animals : A Brief Study Invertebrates

Classification Of Animals : A Brief Study Invertebrates
Classification  Of  Animals

Introduction 

More than 1,000,000 species  of the living animals are already identified and every year some new species are reported. All these animals show a big diversity among them and are classified into different groups  and subgroups.

Major Groups Of Animals

Two common traits in the classification of animals are whether they are made up of a single cell, or of many cells, or whether they have a backbone or have no backbone.

Accordingly, the two major groups or subkingdoms of animals are

1. Protozoa(unicellular or single-celled animals) and

2. Metazoa( animals composed of many cells with formation of tissues and organs)

Next to sub-kingdoms are described the phyla, classes , and orders, etc. As required under the syllabus we describe the classification only up to classes.

Phylum

1. Protozoa.

2. Porifera.

3. Cnidaria.

4. Platyhelminthes.

5. Nemathelminthes.

6. Annelida.

7. Arthropoda.

8. Mollusca.

9. Echinodermata.

10. Chordata.

Classification Of Animals : A Brief Study Invertebrates

Invertebrates Phyla- Protozoa To Echinodermata.

1. Protozoa- These are single-celled animals. Some may form small clusters of cells or colonies. They live mostly in ponds, rivers, lakes, and seas. Some live as parasites on man and other on animals, and often cause diseases. (Example : paramecium, Euglena, Amoeba.

2. Porifera- Porifera are the simplest multicellular animals. The body consists of a  hollow  tube. There is no single mouth but many pores or canals are present in the body wall, through which water enters the body. From this water the sponges  capture their food. A single large opening on the top is the exist for water. The sponges usually have a skleton of microscopic spicules or elastic spongin fibres . Except one or two fresh water sponges all are found in sea. Example : sycon , bathsponges.

3.Cnidaria- In cnidarians a two - layered body wall encloses a single cavity in which digestion takes place. The cavity opens by a mouth at one end only. There cnidarians are found in water. Corals develop hard skeletons made up of calcium carbonate from their secretions. Example : Hydra, jellyfish, corals.

Hydra - Is a tiny freshwater animal found attached to the submerged rocks and plants of quiet lakes, ponds and streams . It has a small hollow cylindrical body with a sticky basal disc for attachment and a crown of about six tentacles at the free end for capturing its prey . The free end also contains a small circular mouth .

Classification Of Animals : A Brief Study Invertebrates

The main body and the tentacles when fully extended may reach a total length of about 1 cm , but on slightest disturbances  the whole animal including the tentacles contracts to a small blot.

Hydra is carnivorous . While waiting for its prey hydra extends its body and tentacles to the largest extent. As soon as any possible victim touches one of the tentacles it is pierced by fine poisonous threads shot out by stinging cells ( nematocysts) located on the tentacles . Held by the threads the paralyzed prey is slowly pushed into the mouth by the combined effort of the contracting tentacles. The ingested prey falls into a large internal space called the gastrovascular cavity where it is digested  and absorbed. 

Platyhelminthes( Flatworms)

These are small, soft , flattened, unsegmented worms without a body cavity . The alimentary canal  has only one opening, the mouth. Most of the flatworms live on or inside other animals as parasites, but a few are free, living in the sea or fresh water. Example: Parasities - liverfluke , Tapeworm.

Nematoda ( Roundworms)

They are long , cyclindrical and insegmented , without a body cavity . The alimentary canal opens at the two ends, mouth and anus . They are mostly parasitic but some live free in the soil. Example: Hookworm and Ascaris in man, celworms in potato plants .

Annelida (segmented or ringed worms)

They body of an annelid worms is cylindrical and divided into ringlike segments. An annelid worm has a well -developed digestive system with the alimentary canal open at the body ends, with a mouth and anus. They have a true body cavity , the coelom . Exampls: Earthworms, Leeches and Neries.

Earthworm

All earthworms live in burrows in  by eating through the soil. The soil is eaten in large quantities and is passed out through the other end of the body in the form of castings. The soil contains organic matter in the form of humus and broken bits of leaves. 

Earthworms avoid light . They are nocturnal. They crawl up on the surface of the ground at night for feeding and mating.

Dry soil is unsuited for earthworms . One main reason for this is that their skin has to be kept moist for respiration. 

Classification Of Animals : A Brief Study Invertebrates

Economic Importance: The earthworms is highly useful in agriculture . It loosens up and aerates the soil as it burrows . This helps in respiration of the roots of plants and makes them grow well and penetrate deeper . The earthworms keep interchanging the top soil with the lower layer thus increasing the fertility of the soil. The earthworm's excreta is rich in nitrogenous matter which is required for plant growth . Presently, earthoworms are being used in vermiculture for producing high-quality manure. Many people use earthworms as baits in catching fish.

Arthropoda (Jointed- legged animals)

This phylum has the largest number of species in the animal kingdom.

These animals have jointed limbs , one pair each on some  or on all body segments. There is an exoskeleton made of chitin. They caste off their exoskeleton during growth in early life, which is regrown. The casting off and the regrowing of the exoskeleton is collectivity called moulting. Examples: Crayfish , Crabs, millepede, centipede, insects.

Arthropods are further divided into four classes

i)  Crustacea 

ii) Myriapoda

iii) Insecta 

iv) Arachnida

i) Class Crustacea are primarily aquatic animals . They have a hard chitinous exoskeleton . They bear two pairs of antennae ( feelers ) and one pair of jaws on the head . The  body  is divided into head, thorax and abdomen, but the head and thorax are fused together . Many appendages are present throughout the body length for various purposes such as feeding, walking, running.

ii) Class Myriapoda- These animals have many segments . Each segment has one or two pairs of limbs. The trunk is not divided into thorax  and abdomen. Examples : Centipede, Millipede ( 2 Pair of limbs in eacg segment).

iii) Class Insecta - Insecta are the most successful animals and form the largesg class of this phylum. Their distinguishing characteristics are as follows:

(1) The body is divided into head, thorax and abdomen.

(2) Those has three pairs of legs.

iv) Class Arachinda - Arachinds have their body divided into two parts , the front  part consists of the head and thorax fused together and the hind part consists of the abdomen.

Mollusca 

These animals have a soft , unsegmented body, with a hard , calcareous shell to protect the soft body. They have a muscular foot to help locomotion and also to act as a weapon in some cases. Example : snails, slugs, oysters, clams.

Echinodermata

Echinoderms are unsegmented , marine animals. They have an exoskeleton and a spiny surface . They move by means of tube- feet . They are radially symmetrical , i.e. they have similar parts arranged regularly around a central point. Examples : Starfish , brittle- stars, sea-urchins, sea- cucumbers.