Stages of Evolution

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The picture below illustrates animal world based on the category to which they belong. The characteristics of animals in each group or category are found to be more closely related as compared to that of other groups. Protozoa are so small that we cannot see them in naked eyes. These were unknown until the invention of microscope.
Porifera:

Commonly called sponges, these are very primitive, multi-cellular animals. They live attached to solid objects in water. Most of the 5000 species of this group live in salt water.

Annelids:

These are 
the type showing the first signs of segmentations. The common earthworms belong to this category.
Mollusks:

The un-segmented body is covered with a thin membrane which secrets a shell. However, these have a complete digestive tract, a muscular ventral foot. Their mouth is equipped with hard, horny teeth for rasping food.
 

Coelenterates:

Coelenterates are so called because they have a hollow tube like intestine. These are flat bodied marine creatures with thread-like tentacles. They also possess a kind of unique, special cells which are used two purposes. One is for making a grip. The other is for stinging with a paralyzing venom. This category include hydra, jellyfish and sea anemone.

Arthropods:

There are more species of animals in this group than in any other. Perhaps nearly 1,000,000 variety is found to belong to this category. One primary feature common to all is the jointed leg. This is absent in any other group. Thus, the butterfly, or the mosquito like insects; the crab-like crustaceans, the spider-like arachnids - all are member to this group.
 
Echinodermata:

These are marine animals having flat spread out bodies with a center. The bodies may or may not be segmented. However, all have a skeleton which is usually flexible with plates like bone joints.

Chordata:

This group belongs to the highest rung of the life pyramid. It has a hollow, dorsal nerve cord and a  well defined heart. 
It includes the variety from fish, reptiles and birds to amphibians and mammals. 

back to life | prehistoric animals | land variety | the mariners

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