THE PROCTUCHA. 



165 



cles are developed on the head in front of the prae-oral ciliated 

 band. But, as development advances, the segmentation be- 

 comes obliterated, the ciliated bands and the feelers vanish, 

 and the worm assumes the characters of a Nemertean. 1 



Fig. 3' 



Fig. 37. 



Fig. 36. 



Fig. 35-37.— Pilidium gyrans (after Leuckart and Pagenstecher). 



35. Young Pilidium : a, alimentary canal ; 6, rudiment of the Nemertean. 



36. Pilidium with a more advanced Nemertean. 



37. Newly-freed Nemertean. 



In species of the genus Xdneus, the ciliated embryo which 

 leaves the egg is speedily converted into a body like a helmet 

 with ear-lappets, and having a tuft of cilia in place of a plume 



1 It is very probable, however, that this larva belongs to the genus Polygor- 

 dius, which appears to be an annectent form between the Turbellaria and other 

 groups. See Schneider, " Ueber Bau und Entwickelung von Polygordius." 

 (" Archiv fur Anatomie und Physiologie," 1868.) 



