230 GENERAL ZOOLOGY 



just beneath the skin (Fig. 123). Fig. 121 shows the posi- 

 tion of the nerve ring just beneath the ring canal. The 

 circular blood-vessel lies between the ring canal and the 

 nerve ring. 



Reproduction and Development. The sexes of the star- 

 fishes are separate, but externally there is no difference be- 

 tween them. At the time the dissections were made for 

 the drawings, the sexual glands in the specimens were small 

 (Figs. 122, 123). As the first of June approaches, the ten 

 sexual glands increase in size until they occupy all the 

 available space in the body cavity of the arms. The ovaries 

 of the females, when they contain ripe eggs, are bright orange 

 in color ; the spermaries of the males are a light cream color. 

 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 be- 

 tween 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 blastula stage (see page 134) is swimming 

 about at the surface by the aid of cilia. The most striking 

 incident in the development of a starfish is the change that 

 takes place after about three weeks of life as a free-swim- 

 ming larva, when it settles toward the bottom and fastens 

 temporarily to a seaweed. 



Until this time the body of the larva has been bilaterally 

 symmetrical, showing no evidence whatsoever of the radial 

 arrangement characteristic of the adult. 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. 122). This 



