DISCOPHORA. 351 



General history of the larva. 



The larva of Clepsine, at the time when the mesoblastic 

 bands have met along the ventral line, is represented in fig. 

 158 B. It is seen to be already segmented, the process having 

 proceeded pari passu with the ventral coalescence of the meso- 

 blastic bands. The segments are formed from before backwards 

 as in Chaetopoda. The dorsal surface is flat and short, and the 

 ventral very convex. The embryo about this time leaves its 

 capsule, and attaches itself to its parent. It rapidly elongates, 

 and the dorsal surface, growing more rapidly than the ventral, 

 becomes at last the more convex. Eventually thirty-three post- 

 oral segments become formed ; of which the eight last coalesce 

 to form the posterior sucker. 



The general development of the body of Nephelis and 

 Hirudo is nearly the same as that of Clepsine. The embryo 

 passes from a spherical to an oval, and then to a vermiform 

 shape. For full details the reader is referred to Robin's 

 memoir. 



The presence of a well-marked protuberance above the 

 oesophagus, which forms the rudiment of a prae-oral lobe, has 

 already been mentioned as characteristic of the embryo of 

 Nephelis ; no such structure is found in Clepsine. 



History of the germinal layers and development of organs. 



The epiblast. The epiblast is formed of a single layer of 

 cells and early develops a delicate cuticle which is clearly formed 

 quite independently of the egg membrane. It becomes raised 

 into a series of transverse rings which bear no relation to the 

 true somites of the mesoblast. 



The nervous system. The nervous system is probably 

 derived from the epiblast, but its origin still requires further 

 investigation. The ventral cord breaks up into a series of 

 ganglia, which at first correspond exactly with the somites of 

 the mesoblast. Of these, four or perhaps three eventually coal- 

 esce to form the sub-cesophageal ganglion, and seven or eight 

 become united in the posterior sucker. 



It would appear from Biitschli's statements that the supra- 



