352 NEPHELIS. 



oesophageal ganglion arises, as in Oligochaeta, independently of 

 the ventral cord. 



Mesoblast. It has already been indicated that the meso- 

 blast 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 im- 

 perfectly known. They become segmented from before back- 

 wards. 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 

 Oligochaeta. 



In Clepsine the mesoblastic bands are stated by Kowalevsky to be- 

 come split into somatic and splanchnic layers, between which are placed 

 the so-called lateral sinuses. These sinuses form, according to Whitman, 

 a single continuous tube investing the alimentary tract ; a tube which 

 differs therefore to a very small extent from the normal body cavity of 

 the Chaetopoda. The somatic layer of mesoblast no doubt gives rise to 

 the circular and longitudinal muscular layers of the embryo. The former 

 is stated to appear the earliest, while the latter, as in the Oligochaeta, 

 first takes its origin on the ventral side. 



A delicate musculature, formed mainly of transverse but also of longi- 

 tudinal fibres, would appear to be developed independently of the meso- 

 blastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and 

 Biitschli). It develops apparently from certain stellate cells which are 

 found between the walls of the alimentary tract and the skin, and which 

 probably correspond to the system of contractile fibres which pass from 

 the body wall to the alimentary tract through the segmentation cavity in 

 the larva of Chaetopoda, various Vermes and Mollusca 1 . 



The mesoblast, so far as is known, gives rise, in addition to 

 the parts already mentioned, to the excretory organs, generative 

 organs, vascular system, etc. 



Excretory organs. There are found in the embryo of Nephe- 

 lis and Hirudo certain remarkable provisional excretory organs 

 the origin and history of which is not yet fully made out. In 

 Nephelis they appear as one (according to Robin, No. 364), or 

 (according to Biitschli, No. 359) as two successive pairs of 



1 According to Robin this system of muscles becomes gradually strengthened and 

 converted into the permanent system. Rathke on the other hand states that it is 

 provisional, and that it is replaced by the muscles developed from the mesoblastic 

 somites. It is possible to suppose that it may really become incorporated in the latter 

 system. 



