X. STARFISH 



In the previous chapters on Hydra and Obelia, mention was 

 made of the radial symmetry exhibited by these forms. Nature's 

 experiments with this plan of animal organization apparently 

 reached its climax in the phylum Echinodermata, a typical repre- 

 sentative of which is to be found in the Starfish. The Coelenter- 

 ates and Echinoderms were, for a long time, placed in the same 

 group, which was known as the Radiata, but later it became 

 evident that they should be placed in separate phyla. 



As a matter of fact it is generally believed that the radial sym- 

 metry of the adult Starfish and other Echinoderms has a more or 

 less superficial character for, in the embryonic condition of these 

 animals, the body plan is not radial but two-sided, that is, a right- 

 and left-sidedness, a condition known as bilateral symmetry. 

 As will be noted in the following chapters, bilateral symmetry is 

 common to practically all the higher types of animals. Considered 

 from various standpoints the Echinoderms appear as an atypical 

 group and one which presents many difficulties with respect to their 

 true relationships. 



The Starfish presents, as will be seen from the description below, 

 a much higher degree of differentiation and complexity in its 

 general structural features than do the animal types heretofore 

 studied. And for the most part it is difficult to relate these Star- 

 fish structures with those present in other animal groups. In other 

 words, we have here an animal exhibiting a number of interesting 

 and complex features which clearly are off the beaten track of 

 animal organization. In correspondence with the unusual struc- 

 tural features are various functional adjustments, some of the most 

 interesting of which are centered around the water vascular system. 



A. External Structure 



The body of the Starfish consists of a central, circular disc from 

 which five, pointed arms radiate. Practically the entire surface 

 of the body is covered with a thin, ciliated epidermis, underneath 

 which is a heavy body wall, studded with blunt, projecting, cal- 



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