56 



EMBRYOLOGY 



In the fui'ther course of development the larva, hitherto 

 spherical, undergoes a flattening, and at the same time the 

 entoderm sac becomes depressed. Then the velum (Fig. 21 

 B, v) is developed from the ring-like wall of the ectoderm. 

 By the enfolding of the area surrounding the mouth-opening 

 the beginning of the sub-umbrellar cavity is established 

 (Fig. 22), which soon increases in size. Since the flattened 

 gastral cavity likewise undergoes an enfolding, it now has 

 the form of a double- walled cup inverted over the sub- 

 umbrellar cavity. According to Brooks, its two walls (in 

 Liriope) come together and fuse with each other at four 



Fig. 22. — Larva of Lirio-pe scuiigeva (after Bbooks). i, interradial area of fusion 

 of the peripheral intestine (cathammal plate); r, radial vessel ; g, circular vessel; 

 t', primary perradial larval tentacle, migrated upward ; t^, interradial larval ten- 

 tacle ; (ä, bud of a permanent perradial tentacle. 



interradial places (Fig. 22 i), thus forming the cathammal 

 plates of the vascular lamella, Avhile the regions that 

 remain unfused repi'esent the four, at first very broad, 

 radial vessels (r) and the ring-canal (gr). The fui-ther 

 changes, through which the larva approaches the structure of 

 the adult, consist in the establishment of the interradial (<^) 

 and the permanent perradial {t^) tentacles (while the primary 

 tentacles disappear), the development of otocysts, the out- 



