STARFISH, SEA-URCHINS, AND SEA-CUCUMBERS. Al 



of the arms are true eyes, since, when shut up in a dark tank, 

 it will crawl towards the light admitted by a crack. The star- 

 fish can also smell its food, for if a bit of a clam is held with 

 a pair of forceps near the creature and slowly withdrawn, the 

 starfish, if very hungry, will crawl after it. The sense of 

 smell is distributed along the lower side of the arms. It is 

 also thought that little bodies on the shells of Echini, called 

 " Sphaeridia," have the combined sense of taste and smell. 



The Starfishes.— These include the snake-star (Fig. 47), 

 and the common five-finger, Asterias vulgaris (Fig. 48). 

 This and allied kinds are 

 abundant on mussel and 

 oyster beds, being very in- 

 jurious to the latter, which 

 serve them as food. The 

 starfish projects its capa- 

 cious stomach between the 

 open valves of the oyster, and 

 sucks in the soft parts, in 

 this way doing much damage 

 to the oyster-beds of the 

 southern coast of New Eng- 

 land. 



All starfishes grow from eggs. After swimming about 

 as a little ciliated sac (gastrula), arms grow out, and it 

 appears much as in Fig. 49. 



The young or larva has now both sides of the body alike. 

 At this time two lobes arise from each side of the mouth. 

 These separate from their attachment and form two dis- 

 tinct hollow cavities; and by the time the larva attains what 

 is called the Brachiolaria stage, the development of the 

 body of the starfish begins, for these two cavities subse- 

 quently develop into two water-tubes. On one of these 

 cavities the back of the starfish is afterward developed, 

 while on the other the under side with the feet or tentacles. 

 Fig. 49 shows the young starfish growing on the posterior 

 end of the larva or young, whose body it is now beginning 



Fio. 47.— Snake-starflsh. 



