ECHINODERMATA 



555 



in the sea or in brood pouches. Cleavage (radial, Fig. 185, i), is total 

 and forms a hollow, one-layered blastula (Fig. 413 A). This, by invagi- 

 nation or unipolar ingrowth, forms a gastrula with a wide blastocoele 

 into which typical mesenchyme cells wander from the wall of the 

 archenteron. The blastopore becomes the anus, and the mouth is 

 formed by the breaking through of a stomodaeum. Meanwhile the 

 archenteron has budded off, at the anterior end, a vesicle which, by 



z9i\ 



-ett. 



1,- end. 



^|3 \'^'^"i^-'~-i=p—arch. 



Up 

 B 



,<^#i^rffe. 



-hcilM. 



-stom. 



D 



\h 



Fig. 413. Stages in the development of Asterias vulgaris. After Field. 

 A, Section of blastula. B, Section of gastrula. C, Section of older gastrula. 

 D, Three days' larva from the right-hand side. An. anus; arch, archenteron; 

 blc. blastocoele ; hip. blastopore ; cil.bd. ciliary band ; coe. rudiment of coelom ; 

 ect. ectoderm; end. endoderm; e?it. enteron; mch. mesenchyme; mth. meso- 

 thelium; stom. stomodaeum. 



processes that differ in detail in different cases, will give rise to the 

 three segments of the coelom described above (p. 551). The future 

 ventral side of the larva becomes concave. The larva is now known as 

 the Dipleurtila. The cilia which uniformly covered the blastula be- 

 come sparse over most of the body but, except in the Crinoidea, grow 

 stronger and more numerous in a longitudinal band around the ventral 

 concavity. This band is the organ of locomotion. Growing more 

 rapidly than the rest of the ectoderm, it becomes thrown into folds, 



