ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 269 



ractol eaoco 



.anus 



pyloric stomach 



cordioc stomoch 



apo't'k caecum 



branehio 

 spine 



Fig. 12-21. Starfish cut in such a manner as to show the internal anatomy. 



their calcareous spiny exoskeleton and com- 

 plex water vascular system as well as other 

 more advanced systems, they have been 

 placed in a separate and much more ad- 

 vanced group. 



The echinoderms are marine animals, 

 mostly free-living but slow-moving. Some 

 are permanently attached forms living at 

 the bottom of the sea; others are commonly 

 found along the seashore, in the sand or on 

 rocks. Very often starfish, which are the 

 most common members of the group, in- 

 vade oyster beds and cause a great deal of 

 damage because these choice morsels are 

 among their chief sources of food. At one 

 time the damage to commercial oyster beds 

 was so great that the problem was indeed 



menacing and costly. Men working in the 

 ovster beds tried in vain to remove the star- 

 fish and destroy them by cutting them in 

 two. This was no solution to the problem 

 for, like lower forms of animals, the starfish 

 has tremendous powers of regeneration. 

 Thus, instead of being destroyed by these 

 measures, the starfish were actually in- 

 creasing; in place of one starfish, the two 

 pieces grew into two new individuals. 



The echinoderms are characterized 

 chiefly by their spiny outer covering, the 

 spines varying from those of microscopic 

 size to the large movable spines found on 

 such animals as the sea urchin. They re- 

 semble the next phylum, the chordates, by 

 the presence of a mesodermal endoskele- 



