3i6 



PHYLUM ECHINODERMA 



endoderm into the segmentation cavity ; (b) from the 

 outgrowing of one or more coelom pouches (vaso-peritoneal 

 vesicles) from the gastrula cavity or archenteron. From 

 these vesicles the body cavity and the rudiments of the 

 water vascular system arise. 



The larva is, first of all, a slightly modified, diffusely 

 ciliated gastrula. In Holothuroids, Echinoids, Asteroids, 

 and Ophiuroids, it becomes quaintly modified b}^ the 

 outgrowth of external processes, and the formation of 

 special ciliated bands. These are at first simply pre-oral 

 and pre-anal rings, but they become drawn out along 



Fig. 170. — Stages in development of Echinoderms. — After Selenka. 



I. Section of blastula of Synapta digitata (Holotluiroid), with a hint of 

 gastrulation. 2. Section of gastrula of Toxopneustes brevispinosus (sea- 

 urchin) ; ec, ectoderm ; en., endoderm ; m., segmentation cavity with 

 mesenchyme cells in it. 3. Section of larva of Asterina gibbosa (star- 

 fish) ; BL, blastopore ; g., archenteron ; v.p., vaso-peritoneal vesicle ; 

 r. and /., right and left sides. 



variously disposed and shaped processes. The larva of 

 Crinoids (of Antedon) is not so divergent. In all cases 

 the bilateral symmetry is preserved. 



The larva does not grow directly into the adult. On the 

 contrary, the adult arises, for the most part, from new 

 growth within the larva on one side. The arms or pro- 

 cesses peculiar to the larva are absorbed or in part thrown 

 off. Only in a few forms which have brood-chambers or 

 are viviparous is the development direct, and without free- 

 swimming larvae. 



LarvaB of Echinoderms. — Johannes Midler was the first to show 

 thai the various types of Echinoderm larvae might be derived from one 

 fundamental form. 



" This fundamental type is an elongated, oval, or pear-shaped larva, 



