ON VERMICULUS LIMOSUS. 



107 



middle of segment X, curves slightly towards the median plane and opens into 

 the atrium. The latter has a spacious ellipsoidal cavity and opens slightly in 

 front of the sete directly into the common sperm-duct chamber below the ventral 

 cord, which is, as Goodrich says, to be regarded as an invagination of the 

 body-wall. In immature individuals the atrium is followed by a slender duct- 

 like portion, but as the genital organs approach maturity the invagination of 

 the body-wall becomes greater and the dorsal wall of the duct-like portion is 

 converted into the roof of the sperm-duct chamber, and the atrium comes to 

 open directly into the latter. Even in mature specimens the sperm-duct cham- 

 ber is sometimes very small and the duct-like continuation of the atrium persists* 

 The internal surface of the funnel as well as of the duct is lined with ciliated 

 epithelium, but in the atrium the cilia are absent and the cells are taller and 

 glandular. The outer surface of the whole organ is covered with peritoneal 

 cells, which are conspicuously taller around the atrium. Between the inner and 

 the outer epithelium there is a layer of circular and longitudinal muscle fibres' 

 which are most strongly developed around the atrium and very thin in the funnel 

 and the duct. There is no penis. 



Alimentary Canal. 



The alimentary canal is simple as in other Tubificidae. The mouth lies on 

 the ventral side of segment I ; the pharynx is large and lies in segment II; the 

 œsophagus is slender and extends through segments III and IV, the intestine 

 beginning in segment V. The lumen of the intestine is about equal to that of 

 the œsophagus, but as the former is surrounded by hepatic cells it appears exter- 

 nally thicker than the œsophagus. On the dorsal side of the pharynx there is a 

 group of goblet-shaped unicellular glands with long necks opening into the basal 

 portion of the ciliated epithelium of the pharynx. The ventral wall of the 

 pharynx is very thick and is concave towards the ventolin. The œsophagus 

 and the intestine are lined by a ciliated epithelium, which is followed by a layer 

 of circular and of longitudinal muscle fibres. The intestinal wall is very rich in 

 blood-vessels. In sections these are seen to be situated between the internal epi- 

 thelium and the layer of circular fibres, and are traversed by connective tissue 

 trabecular 



