702 PROCEEDINGS OF THE ACADEMY OF [DeC. , 



Ing merely of the endothelium. Outside of the membraneous en- 

 dothelium, in the more posterior part of the lateral vessels, fine 

 librils are seen. These may be either fine muscular fibrils, or the 

 end processes of the branched conpective tissue cells that are so 

 numerous around the blood vessels. From their general appear- 

 ance, and from the absence of muscle fibres in the more anterior 

 part of the lateral vessels, I am inclined to regard them rather as 

 connective tissue fibres. 



8. The Excretory System. — The paired nephridia,' fig. 1, 

 Nph., lie in the anterior part of the body, about 6-7 mm. behind 

 the mouth, bordering on the ventral surface of the lateral blood 

 vessels and running parallel with them; they are about 2.5 mm. 

 in length, and there is no communication between the two nephri- 

 dia. Each nephridium may be briefly described as a slightly con- 

 voluted tube (the main duct) that opens to the exterior by a nar- 

 row duct (excretory duct) at its posterior end, and which ante- 

 riorly gives off a number of slender, thin-walled branches (the 

 ductules), each of these ending blindly in a group of specialized 

 cells, known as a terminal l)alb (" Eudkiilbcheu, " Burger). 



The most anterior part of the nephridial system, consisting of 

 the terminal bulbs, fig. 47, T.B., and their ductules, Nph.d., is 

 found along the ventral surface of the lateral blood vessels, L. V., 

 and their ventral connectives, Plate XLI, fig. 22, Y. hi. con. No 

 main duct is present in this region, and the irregular network of 

 the fine ductules and their blind ends at first seemed a hopelessly 

 confused mass of tissue, but by the careful study of serial sections 

 the relative arrangement of the parts has been made out. 



The terminal bulb, Plate XLIII, fig. 47, T.B., is the blind en- 

 larged end of the fine ductule, N2)h.d., coming from the main duct. 

 fig. 46, Npli.D. Each bulb consists of a number of cells, probably 

 eight to ten or more, but it is difiicult to determine this exactly as a 

 bulb does not always lie wliolly in one section. The cells of the bulb 

 are placed side to side about a central lumen, so that the walls of 

 the bulb are one cell thick. The height of the cells — that is, the 



'Several attempts were made to study the nephridia in life, but they 

 could be seen in two specimens only. In these, merely the presence of the 

 main duct was made out, and in one case the excretory dnct. The methy- 

 leu blue metliod, employed ])y Biirger (1892, p. 327, Ibotuote) for the study 

 of the nephridia of Eapoliu and various Metanemerteaus, was tried ])ut 

 without success, so that the following description is based entirely upon the 

 study of sections. 



