290 NEPHELIS. 



bands at the two sides of the body, equivalent to the mesoblastic bands 

 of the Chaetopoda. The three vitelline spheres, still lai'gely uncovered by 

 the epiblast, lie at the posterior end of the body. The embryo grows 

 rapidly, especially anteriorly, and the three vitelline spheres become 

 covered by a layer of flattened epiblast cells. Around the (esophagus a 

 cavity traversed by muscular fibres is established. Elsewhere there is no 

 trace of such a cavity. The cephalic region becomes ciliated, and the 

 dorsal part of it, which represents a rudimentary prse-oral lobe, is especially 

 prominent. The cilia of the oral region are continued into the lumen of 

 the oesophagus, and at a later period are prolonged, as in Lumbricus, along 

 the median line of the ventral surface. 



The mesoblastic bands would seem from Biitschli's observations, which 

 receive confirmation from Kleinenberg's researches on Lumbricus, to be pro- 

 longed dorsally to the oesophagus into the cephalic region. Posteriorly they 

 abut on the large vitelline spheres, which were supposed by Kowalevsky 

 to give origin to them, and to play the same part as the large meso- 

 blasts in Lumbricus. It has already been shewn that the function of the 

 large cells in Lumbricus has been exaggerated, and Biitschli denies to 

 them in Nephelis any share in the production of the niesoblast. It seems 

 in fact probable that they are homologous with the three vitelline spheres 

 of Clepsine ; and that their primitive function is to give origin to the 

 hypoblast. They are visible for a long time at the hind end of the embryo, 

 but eventually break up into smaller cells, the fate of which is unknown. 



The embryo ot Hirudo would appear from the researches of Robin 

 to develop in nearly the same way as that of Nephelis. The anterior 

 part is not however ciliated. The three large posterior cells disappear 

 relatively early. 



General history of the larra. 



t/ / 



The larva of Clepsiue, at the time when the mesoblastic bands 

 have met along the ventral line, is represented in rig. 158 B. It is 

 seen to be already segmented, the process having proceeded pari 

 pass u with the ventral coalescence of the mesoblastic 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. Jt rapidly elongates, and the dorsal surface, growing more 

 rapidly than the ventral, becomes at last the more convex. Even- 

 tually 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 prre-oral lobe, has already been men- 

 tioned as characteristic of the embryo of Nephelis; no such structure 

 is found in Clepsine. 



