THE STARFISH 249 



on the two sides or bilaterally sjinmetrical.' It swims freely 

 ill the water and is known as a larva. The larva has, it will 

 be observed, very little relation in form to the adult starfish. 

 The change from the larval to the adult starfish is known as a 

 metamorphosis. Even in the bilateral stage the system of 

 water-tubes of the adult starfish has begun to form and, finally, 

 over the stomach on the left side five pockets, radially ar- 

 ranged, make their appearance; these are the five radial 

 water-tubes, one of which runs down each future arm. On 

 the right side of the stomach, calcareous plates are laid down 

 to form the aboral wall of the starfish.^ 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. 



The larva now ceases to swim and settles to the bottom. 

 There the process of resorption is completed, the upper and 

 lower surfaces of the star approach each other, the limy skele- 

 ton 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 are acquired at a later stage. 



1 Figs. 2.37, 238. ' Figs. 239, 240, ah, ah'. 



Figs. 237-240. — Larval stages and metamorphosis of starfish. After draw- 

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



Fig. 237. — Dorsal view of starfish larva with beginnings of ciliated arms. 



Fig. 238. — Ventral view of same larva. 



Fig. 239. — Side view of an older larva in which young star is beginning to 

 arise. 



Fig. 240. — Ventral view of same larva, a, anterior end : h, posterior end ; 

 J— 4, ciliated projections; a', preoral arms; am, oral surface of young star 

 with beginning water-tubes, aiii—am^; ah, ah', ah", aboral surface of star; 

 c, postoral arm; i, intestine; m, inouth ; n, lateral arm; o, anus; tjp, 

 oesophagus; q, q^, dorsal lateral arms; .s, stomach; unv' , water tubes; 

 am. Fig. 238, region from which water-tubes arise. 



