112 EDWIN S. GOODRICH. 



coelom, through which passes the alimentary canal, suspended 

 by a median ventral mesentery. This cavity does not com- 

 municate with the exterior; it is limited in front by a 

 complete septum, and behind is, in some forms, but not in all, 

 separated from the body-wall by a perianal sinus. 



In front of the septum are two distinct cavities. The 

 larger of these is the pre-septal h^mococl, a blood-cavity 

 which occupies nearly the whole region from the septum to 

 the edge of the pre-oral hood. Muscular strands and fibrous 

 strands traverse it in many directions, and in the later stages 

 haemoglobinous blood-corpuscles float in the fluid contained 

 in it. The epithelium lining this cavity is not as distinct as 

 that of the coelomic cavities, and in some regions is but 

 irrregularly if at all developed. A distinct pocket, with an 

 epithelial lining, is situated between the brain and the oeso- 

 phagus, and opens ventrally. The subneural space is but a 

 portion of the general pre-septal hasmocoel. Continuations of 

 the pre-septal liaBmocoel extend to the tips of the tentacles 

 and down the mid-dorsal and the mid-ventral lines, forming 

 the blood-vessels lying on the stomach. During metamor- 

 phosis it becomes converted into the ring or lophophoral 

 blood-vessel of the adult. 



Running close beneath the body-wall anteriorly to the 

 septum is the pre-septal coelom. This coelomic space is in 

 the form of a horseshoe-shaped canal, sending a short blind 

 branch into the base of each tentacle. It appears somewhat 

 late, and becomes converted into the pre-septal or lopho- 

 phoral ccelom of the adult. Neither of the pre-septal 

 cavities open to the exterior in the larva. 



We are now in a position to discuss the structure and 

 relations of the nephridium itself. 



The Nkphridia. 



Wagener, in 1847 (18), was the first to describe the 

 excretory organs of Actinotrocha, and his figure of the 

 nephridium, with its bunch of elongated " spermatozoon-like 



