194 ZOOLOGY 



each side of the body, and the larva becomes bilaterally 

 symmetrical. 1 Meanwhile, inside the body, the system 

 of water tubes of the adult starfish has begun to form, 

 and finally, over the stomach on the left side, five main 

 water tubes, radially arranged, make their appearance ; 

 these are the five radial tubes, one of which runs down 

 each future arm. 2 On the right side of the stomach, calca- 

 reous plates are laid down to form the aboral wall of the 

 starfish. 3 The young star is now found as a parasite at the 

 hinder end of the larva. Soon all the front end of the larva, 

 together with the long arms, becomes absorbed. Up to 

 this stage the larva lias been free-swimming, but it now 

 settles to the bottom. There the process of resorption is 

 completed, the upper and lower surfaces of the star 

 approach each other, the calcareous skeleton and sucking- 

 feet rapidly develop. At this stage the disk is so large, 

 and the arms so short, that the young starfish looks more 

 like a sea-urchin than an adult starfish. The elongated 

 arms and the pedicellarue of the adult are not gained until 

 three years have passed. 



t/ -*- 



1 Figs. 176 and 177. 2 Fig. 178. Fig. 179, ab>. 



FIG. 176-179. Larval stages and metamorphosis of starfish. After drawings 

 of E. B. Wilson, from Brooks, " Invertebrate Zoology." 



FIG. 176. Dorsal view of starfish larva with beginnings of ciliated arms. 



FIG. 177. Ventral view of same larva. 



FIG. 178. Side view of an older larva in which young star is beginning to 

 arise. 



FIG. 179. Ventral view of same larva, a, anterior end ; b, posterior end; 

 1-4- , ciliated projections; a', preoral arms; a HI, oral surface of young star 

 with beginning water tubes, am-am 5 ; ab, ub', ab^, aboral surface of star; 

 c, postoral arm ; i, intestine; m, mouth; n, lateral arm ; o,anus; ce, oesopha- 

 gus; q, q', dorsal lateral arms; *, stomach; ww', water tubes ; am, Fig. 177. 

 region from which water tubes arise. 



