2 7 6 



PHYLUM ECH1NODERMA. 



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

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

 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 



FIG. 141. Stages in development of Echinoderms. After Selenka. 



i. Section of blastula of Synapta digitata (Holothuroid), with a hint of 

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

 urchin) ; CO., ectoderm ; cn. t endpderm ; m. t segmentation cavity with 

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

 fish); BL, blastopore ; g. , archenteron ; z>./., vaso-peritoneal vesicle ; 

 r. and /., right and left sides. 



are viviparous is the development direct, and without free- 

 swimming larvae. 



The celebrated comparative anatomist and physiologist, Johannes 

 Miiller, was the first to show that the various types of Echinoderm 

 larvae might be derived from one fundamental form. 



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

 which is somewhat flattened on its ventral side. It has arisen from a 

 gastrula whose blastopore has become the anus, while the archenteron 

 is bent towards the ventral surface, where it communicates by the larval 

 mouth with the exterior. Besides these two apertures, the larva has a 

 third, namely, the dorsal pore of the water-vascular system. The cilia, 

 with which the larva was at first uniformly covered, partly disappear, 

 and persist only in restricted regions or ciliated bands" (Korschelt and 

 Heider). 



Crinoids. The simplest Echinoderm larva is that of Antedon^ a 

 somewhat modified oval, with five transverse rings of cilia (the most 



