382 



EMBRYOLOGY 



manner as that described above, for some species pass 

 through a larval stage, the shape of which recalls the larvse 

 of the Echinodermata. The larva called Tornaria was de- 

 scribed bj JoH. MtJLLER as an Echinoderm larva. Its 

 shape, which, moreover, exhibits modifications in the different 

 species, is illustrated by Fig. 169. On the ventral side of 

 the bell-shaped larva lies the mouth-opening, from which 

 the oesophagus ascends, and then bends backwards, to be- 

 come continuous with the capacious stomach. Upon this 



Fig. 169.—^ and B, Tornaria and later stage of development of Balanoglossus 

 (after Kowalevsky, from Balfocr's Comparative Emhryology). The broad black 

 lines indicate the ciliated band and the ring of cilia behind it. an, anus; hr, 

 gill-pocket; c, body cavity; Ji^, " heart " ; m, mouth ; iv, the so-called water- vas- 

 cular vesicle. 



follows the hind-gut, which opens to the exterior through 

 the anus at the posterior end of the larva. The surface of 

 the larva becomes engirdled by ciliated bands, which, how- 

 ever, are distinguished from those of the Echinoderm larvae 

 by their different parts acquiring a greater independence. 

 In the first place, we distinguish a preoral from a post-oral 



