244 GENERAL ZOOLOGY 



From about the first to the middle of July, in the latitude of 

 Boston, the sexual cells are sent out into the water through 

 ten small holes on the aboral surface, two at each angle 

 between the arms all around. The egg-cells are fertilized by 

 the sperm-cells in the water, and within a few hours the 

 young starfish in the Uastula stage (see p. 214) is swimming 

 about at the surface by the aid of cilia. The manner of 

 development after the yastrula stage (see p. 215) is quite 

 different from the plan described for the earthworm. The 

 most striking incident in the development of a starfish is the 

 change that takes place after about three weeks of life as a 

 pelagic bilaterally symmetrical larva, when it settles toward 

 the bottom and fastens temporarily to a seaweed. On the 

 posterior region a star-shaped bud is formed, which becomes 

 the adult starfish. As the bud grows it draws into itself the 

 larva which gave rise to it. The starfish attains the stage of 

 sexual maturity within a year. 



Regeneration. It is not unusual to find in a lot of star- 

 fishes dredged from the bottom, many specimens that have 

 one or more arms shorter than the others (Fig. 119, 15). 

 This means that some accident has befallen the irregular 

 specimens, and that new arms are being formed to take 

 the place of those that were lost. It has been found that a 

 starfish deprived of all its arms will, under favorable circum- 

 stances, reform all five ; but one that has suffered an injury 

 as extensive as a cut through the entire disk probably never 

 survives. 



THE BASKET-FISH 



Occasionally fishermen bring up on their hooks from the 

 deep waters of sounds specimens of the basket-fish, Astroph'- 

 yton agassiz'ii (Fig. 122). It is very difficult to keep them 

 alive long on account of the change from the great pressure 

 and the low temperature of their habitat. 



