748 PROFESSOR J. STEPHENSON ON 



. . . HuBRECHT bemerkt, class ihm ' der Versuch immer als ein verfehlter erschienen 

 ist, die parasitischen Nematoden sich hervorgegangen zu denken aus den frei im slissen 

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

 freie Existenz adaptiert." 



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

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

 commonly among the aquatic Oligochseta ; the genus Chwtogaster 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. The Contractions of the Alimentary Wall in the Aquatic Oligoch^ta 

 IN relation to those of the Vascular System. 



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

 number of the forms previously mentioned, between the postero-anterior 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 which 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 comj)onent layers of the wall of the intestine, I shall not enter. 



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

 in Enchytrgeids, 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. 



^OLOSOMATID.^i. 



j^olosoma hemiorichi Ehrbg. 



Anatomy of Circulato7"y System. — In the form which I have identified as ^olosoma 

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

 intestinal wall contains a system of vacuole-like lacunae. This lacunar system extends 

 throughout the stomach also, but in this 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 single 

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

 this becomes a definite blood-vessel with a clear uninterrupted lumen, extending 



