THE STARFISH AND ITS ALLIES 193 



Food. The favorite food of the starfish is some kind of 

 mollusk, especially bivalves, such as mussels and oysters. 

 Their method of feeding is as follows: They crawl over 

 the bivalve, arch the body over it, apparently pull open 

 the valves, then turn the stomach inside out over the soft 

 body within the shell, and gradually digest it. The star- 

 fish probably finds its food by means of a keen chemical 

 sense. 



Starfishes are of economic importance on account of their 

 destructiveness to oysters, but since the oystermen have 

 learned to keep the beds clear of starfishes and that tear- 

 ing starfishes into pieces only aids in their increase, as 

 each piece produces a whole starfish, the damage wrought 

 by starfishes must be less than it was formerly. 



Development. Starfishes have the sexes distinct ; and 

 the sex can usually be told by the color. The female has 

 a more bluish tint, while the males are of a reddish brown 

 color. Both the eggs and sperm are extruded into the 

 water through small openings on the aboral surface at the 

 interspace between two adjacent arms. The egg is hardly 

 visible to the naked eye, and is enclosed in a delicate 

 membrane. The egg undergoes cleavage ; that is to say, 

 the single sphere breaks up into two, four, eight, sixteen 

 spheres, and so on. Finally a hollow ball, made up of 

 these spheres or cells, is formed, called the "blastula." 

 Next the wall of this hollow sphere is pushed in at one 

 side, forming a sort of cup with two walls. This is the 

 so-called "gastrula." The cavity of this cup is the 

 digestive cavity. At first it is a sac with only one 

 opening, but later a second opening, the true mouth, 

 breaks through, and the former opening persists as the 

 anus. Pairs of arms, edged with cilia, now bud out on 

 o 



