748 PROFESSOR J. STEPHENSON ON 



. . . HiTBRECHT bemerkt, dass ihm ' der Versuch immer als ein verfehlter erschienen 

 ist, die parasitischeii Nematoden sich hervorgegangen zu denkeii aus den frei im sussen 

 Wasser, im Meere und in der Erde lebenden.' Letztere hiitten sich sekundar an die 

 freie Existenz adaptiert." 



Summing up the present section, we may conclude that phenomena whicli point 

 to the intestine as a respiratory organ of some importance occur very 

 commonly among the aquatic Oligochieta ; the genus Chietogaster forms a 

 remarkable exception to the general rule — an exception which it is proposed 

 to explain by assuming the descent of existing forms from endoparasitic 

 ancestors. 



2. Thk Contractions of the Alimentary Wall in the Aquatic Oligoch.^ta 

 IN relation to those of the Vascular >System. 



I propose in tlie present section to consider the relations which exist, in a certain 

 number of the forms previously mentioned, between the postero-antei'ior contractions 

 of the alimentary wall and the postero-anterior contractions of the intestinal vascular 

 network, or of the dorsal vessel of the circulatory system. For this purpose I shall 

 take the various forms in the order whicli seems to me best calculated to bring out 

 what I consider to be the meaning of the phenomena described. 



It will be necessary to mention the chief features of the circulatory .system in a 

 number of these forms ; but into the much-discussed questions of the histology of the 

 walls of the blood-channels, and of the exact situation of these latter in respect of 

 the several component layers of the wall of the intestine, I shall not enter. 



Vejdovsky (53), in describing the antiperistaltic movements of the intestinal wall 

 in Enchytrajids, describes also how these movements propagate themselves forwards, 

 beyond the point where the dorsal vessel arises from the intestinal sinus, as the postero- 

 anterior contractions of the dorsal vessel. I have not met with any other observations 

 of a similar nature. 



.Eolosomatid.*;. 



jEolosoma hemprichi Ehrbg. 



Anatomy of Circulatory System. — In the form which I have identified as ^■Eolosoma 

 hemprichi (46) there is no separate dorsal vessel in the region of the intestine, but the 

 intestinal wall contains a system of vacuolc-like lacumt. This lacunar system extends 

 throughout the stomach also, but in tlii.s region a small though distinct dorsal vessel 

 makes its appearance, the cavity of which is traversed by strands. The vascular 

 channels are continued forwards dorsally on the oesophagus by a wide, thin-walled vessel 

 which appears as if made up of a number of apposed vacuole-like chambers, or as a sinf^le 

 elongated chamber traversed by numerous strands or septa. In front of the oesophagus 

 this becomes a dclinite blood-ve.ssel with a clear uninterrupted lumen, extending 



