122 DR. J. STEPHENSON ON THE MORPHOLOGY, CLASSIFICATION, 



of the oval sac, is straight and stiff, and- makes a right angle or 

 rather less with the general direction of the sac (except where 

 the duct has the lower opening, v. inf.). 



In whole preparations of the nephridium the ectal end, where 

 it is broken off from the body-wall, always appears trumpet- 

 shaped, the end of the trumpet being oblique and the dorsal lip 

 the more prominent {tr., fig. 1). The duct is covered by a layer 

 of cubical peritoneal cells ; the remaining layers are the same as 

 those of the oval sac, but all are thinner ; the lining epithelium 

 consists of approximately cubical slightly staining cells. 



The duct is attached by a narrow mesentery to the posterior 

 face of the septum. Reaching the angle between the septum 

 and the parietes it dilates somewhat, the dilatation corresponding 

 to the trumpet-shaped ending of the duct as seen in the isolated 

 nephridium (v. sup.). 



The duct may reach the body- wall in one. of two situations — 

 either at the level of the lateral or of the ventral setse ; in the 

 first case it makes an angle ra,ther less than a right angle with 

 the axis of the oval sac, while in the second it appears as a direct 

 downwai'd continuation of the sac. . At both these levels the 

 long•it^dinal muscular coat shows an interruption in transverse 

 sections;- the trximpet-shaped dilatation rests therefore on the 

 cii'cular muscular coat. 



If the duct has the lower position, it penetrates the parietes 

 forthwith ; the blue-staining layer becomes continuous with the 

 similar layer of the body-wall, and the epithelium of the duct 

 with the superficial epithelium of the body. The same may 

 happen if the duct reaches the body-wall at the level of the 

 lateral sette — the duct may penetrate the parietes immediately, 

 in a slightly upward direction. Often, however, the duct 

 continues in the body-wall in an upward direction, and reaches a 

 level not very far from the mid-dorsal line— a level corresponding 

 approximately on the dorsal to that of the ventral setee on the 

 ventral side— before it comes to the surface. 



In the latter case the muscular layer of the duct ceases to be 

 distinguishable; the blue-staining layer becomes continuous with 

 a very marked similarly staining layer between the circular and 

 longiti^dinal muscular coats of the body- wall ; and the epithelial 

 layer of the duct is only distinct at the beginning and end of its 

 course within the wall.. For- .the- greater joart of this course the 

 duct appears rather like a. split in the . body-wall immediately 

 internal to the circular muscular layer ; the epithelial layer is 

 difllcult to make out (except at the two ends), and the lumen of 

 the duct appears in most parts to be bounded by a vei-y definite 

 wall of the blue-staining connective tissue. • 



I give below the positions of the endings of the nephridial 

 ducts in a consecutive series of eleven segments, both on the 

 right and left sides. . It will be noted that there are no apertures 

 in the lowest position (on the level of the ventral setse) on the 

 left side and none in the dorsal position on the right side. 



