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 mesoblastic bands. The segments are formed from before backwards as in Chætopoda. 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 præ-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 coalesce to form the sub-oesophageal ganglion, and seven or eight become united in the posterior sucker. It would appear from Bütschli's statements that the supra-oesophageal ganglion arises, as in Oligochæta, independently of the ventral cord. Mesoblast. It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine from the two mesoblastic bands which unite in the median ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually grow upwards and meet in the dorsal line. Septa are formed between the somites probably in the same way as in the Oligochæta.