68 PART I. GENERAL ACCOUNT 



There are important differences in the topography of the excretory system 

 and in the degree of development of the coelomoducts between the Athe- 

 canephria and the Thecanephria. In the Thecanephria the coelomoducts are 

 comparatively short, lying wholly within the protosoma, and their excretory 

 sections are closely apposed to the lateral cephalic vessels. They are far bigger 

 in the Thecanephria, extending back beyond the hind margin of the proto- 

 soma; their excretory portions are almost median and enclosed within a 

 sac-like outgrowth of the ventral wall of the dorsal blood vessel (Ivanov, 

 1955a). 



As a typical athecanephrian we may select Oligobrachia dogieli for descrip- 

 tion. In this species the U-shaped excretory portion of the coleomoduct is 

 very broad and lies on the ventral surface of the lateral cephalic vessel. The 

 discharge branch is short and the external pore is located on the ventral side 

 of the body behind the tentacles (PI. II, facing p. 68). 



The coelomoducts of Siboglinum cauUeryi have essentially the same struc- 

 ture. The excretory function, however, is carried out not only by the U- 

 shaped part of the organ but by a considerable part of the proximal canal 

 also and by the anastomosing duct which joins the proximal canals of 

 opposite sides. The coelomoducts closely invest the median cephalic vessel on 

 the dorsal and lateral sides, and then turn outwards to abut upon the lateral 

 cephalic vessels. Running from the protocoele the proximal canals gradually 

 broaden then turn up onto the dorsal side where they communicate with one 

 another by means of a wide sac-like anastomosis. Then the ducts become 

 narrow again and closely invest the median cephalic vessel. The U-shaped 

 part of each coelomoduct lies on the lateral cephalic vessel. Its distal section 

 gradually runs into the discharge canal. The ampulla is situated on the ventral 

 side of the body (PI. I, facing p. 54). 



The coelomoducts of the Thecanaphria, as stated above, are very much 

 larger and their excretory portions are strongly developed. The internal ends 

 of the coelomoducts are much elongated, making numerous bends and loops 

 (Fig. 47), and the surface of the excretory epithelium is thus considerably 

 expanded. The proximal canals, opening from the protocoele, form charac- 

 teristic expansions at the point of the junction with the anastomosing trans- 

 verse duct. The ampullae of the distal canals open on the dorso-lateral surface 

 of the protosoma at the very base of the tentacular crown. The sole exception 

 to this pattern, to all appearances, is Lamellisabelhi zachsi in which the coelo- 

 moducts do not communicate at all with the coelom (see PI. HI, facing p. 92). 



The histological structure of the coelomoducts is the same in all species. 

 No ciliated tract runs directly from the coelom into the proximal canals, but 



