28 



the rectum. A circle of long cilia is placed on the praeoral 

 lobe immediately in front of the mouth, and there may be 

 also a perianal and several other post-oral bands of short 

 cilia. There is often an epiblastic thickening of a nervous 



Fig. 14. Trochosphere. p, prae-oral lobe ; q, post-oral or abdominal part of 

 body ; c, prae-oral circle of large cilia ; p.c, perianal circle of cUia ; m, mouth ; 

 oe, oesophagus (stomodaeum) ; s, stomach ; i, intestine ; a, anus ; g, supra-oesopha- 

 geal ganglion ; o, eye-spot. 



nature in the prae-oral lobe, and simple sense organs may be 

 developed in connection with it. Paired excretory organs 

 may also be present in the form of small tubes opening to 

 the exterior and communicating with the body cavity which 

 surrounds the alimentary canal. 



In the Echinodermata the typical larval form is charac- 

 terised by having no ganglion or sense organs on the prae-oral 

 lobe, and by the chief band of cilia being post-oral in posi- 

 tion. In these respects it differs from the Trochosphere, and 

 hence it has been argued* that the Echinodermata and the 

 Mollusca cannot have had a common point of origin, but 

 Tornaria, the larva of Balanoglossus (Enteropneusta), shows 

 in some respects characters which are intermediate between 

 those of the Echinoderm Bipinnaria larva and those of the 

 Trochosphere. Balfour, however, has suggested that the 

 resemblances of Tornaria to the Trochosphere are adaptive, 

 and do not indicate close genetic affinity. If this should 



* Comp. Emb. vol ii, p. 316. 



