ECHINODERMATA 153 



a nerve to each arm ; and (3) the coelomic, the nerves extending 

 along the top of each arm. 



The sense organs include that of smell and the so-called eye. 

 At the tip of each arm is a bright red speck, the eye-spot. This 

 is practically nothing but a collection of pigment granules upon 

 the expansion of nerve cord ; since there is no lens there can be 

 no image, and hence the animal can probably distinguish light 

 and color, but not form. The perception of light is probably 

 merely the sensation of warmth due to the absorption of the 

 light rays by the pigment and their consequent conversion into 

 heat. 



Over the eve is a tentacle similar to a tube-foot, but smaller 

 and without the sucker. Experiments have determined its 

 function to be that of smell, and that the animal is guided to its 

 food more by this sense than by sight. 



Reproduction is exclusively sexual. The eggs and spermatozoa 

 are discharged into the water in great abundance during the last 

 three weeks of June, although they are also found during the 

 summer and occasionally even during the winter. The eggs 

 after fertilization develop into little transparent larvae covered 

 with waving cilia. At this time their greatest enemy is the 

 menhaden. They swim about slowly near the surface and feed 

 on minute organisms until they attain a length of an eighth of 

 an inch. Then the star shape begins to develop and in a few 

 hours a very small star lies at the bottom of the ocean. These 

 settle on seaweeds and eel grass and begin at once to devour the 

 young clams which have likewise begun life here. It has been 

 found that one of these little starfishes devoured over fifty young 

 clams somewhat smaller than itself in six days. Their growth 

 depends upon the amount of food eaten, and hence the age at 

 which a female begins to produce eggs is said to vary from one 

 to six years. A starfish can regenerate lost arms. 



I. Sketch (a) ventral view, (b) dorsal view. Label disk, 

 arms, ambulacra, tube-feet openings, mouth, eye spots, mad- 

 reporite, anal plate, spines. 



