TROCHOSPHERE LARVA 



263 



worm-types, and also in Mollnscs. Its chief characters are the 

 following : — 



(i) There is a prominent pre-oral region, with an apical ganglion and 



a girdle of cilia. 



(2) The gut has a distinct ventral curve, and a threefold origin. 



Fig. 142. — Diagram showing structure of a Trochosphere of 

 Etipomatus. — After Shearer. 



M., Mouth; A., anus; S., stomach; A^., larval kidney; 

 S.O., apical sense organ ; E., eye-spot ; ME., beginnmg of 

 mesoderm; A.V., anal vesicle. 



(3) The larval cavity is the persistent segmentation cavity, and in it 

 posteriorly lie the primitive mesoderm cells. 



The Trochosphere is a free-swimming pelagic larva, which, among 

 worms, corresponds largely to the future head region of the adult. 

 The change to adult form probably takes place in the most primitive 



