226 



HOW DO WE CLASSIFY ANIMALS? 



Practical Exercise 4. Study the diagram on page 224 and construct a cross 

 section of the animal. Label all parts shown. 



HydCro*p<x 



Ofoelia v 



sea ai?$£mon<2, 



m " 



|coelenterata| 



Sc^pVzo^ocx / > Ctenophora 



#.,; 



jellyfish. 



comb jelly 



Class I. Hydrozo'a. Simple animals as hydra, or colonial in habit as the hydroids. They 

 produce new individuals by budding, and the eggs and sperms are usually produced in a free- 

 swimming jellyfish, which buds off from the original colony. This is an example of alterna- 

 tion of generations. Examples : Hydra and Obe'lia. 



Class II. Scyphozo'a. Marine jellyfish, mostly of large size. Example, Aurelia. 



Class III. Anthozo'a. Hydralike animals, usually attached, with many tentacles, disposed 

 in circlets in multiples of five. They may be single or colonial. The sea anemones and 

 corals are the best-known examples. 



Class IV. Ctenophora, or sea walnuts, well known along our eastern coast, are sometimes 

 given as a separate phylum and sometimes as a class of the coelenterates. 



Jellyfish. At first sight you would not say a jellyfish was re- 

 lated to the Hydra, but we find that a part of the life of the 

 jellyfish is passed as a colony of hydralike animals which give 

 rise to free-swimming jellyfish as the sexual stage of their life 

 history. This alternation of an asexual generation with that 

 of a sexual generation, which produces the eggs and sperm 

 cells, is seen in many plants and is best shown in this group of 

 animals. 



Echinoderms. 1 These are spiny-skinned animals which live in 

 salt water. They show radial symmetry. There are about 4500 

 named species. 



The starfish. By far the most important enemy of the oyster 

 and other salt-water mollusks 2 is the starfish. The common 

 starfish, as the name indicates, is shaped like a five-pointed star. 

 A skeleton of lime which is made up of thousands of tiny plates. 



1 Echinoderm : (e-kl'no-durm). 



2 Mollusk : popularly called shellfish. 



Has soft body protected by shell. 



