354 MOORE. [Vol. XIII. 



dorsal pore by which the embryonic head kidneys of Lumbricus 

 open. In the simpler and more primitive condition exhibited 

 by Branchiobdella and in the posterior nephridia of other genera 

 the vesicle is of small size, less than twice the diameter of the 

 efferent duct, and of a depressed globoid shape. The external 

 pore is either terminal, as in the anterior pair of B. instabilia, in 

 which case the vesicles are somewhat pointed, or near the middle 

 of the external face. In all cases the vesicle (Figs. 40-42) 

 consists of a simple invagination of the epidermis with its 

 cuticular covering, forming the lining epithelium and the greater 

 thickness of its walls. External to this is a thin connective- 

 tissue sheath with small nuclei (ct) and fibrous strands running 

 to the adjacent muscles. The vesicle is closely related to the 

 layer of circular muscle fibres, between two of which the 

 external pore perforates the skin ; but whether a muscular coat 

 is present on the vesicles themselves, as in the larger anterior 

 one of Bdellodrilus, is doubtful. Sections (Fig. 40) do not 

 indicate the existence of one, and if such be present it must be 

 very thin indeed. A delicate sphincter surrounds the external 

 pore, which may be circular, or wrinkled, or slit-like, or com- 

 pletely closed, according to the degree of contraction of this 

 muscle. I have frequently watched the anterior vesicles of B. 

 instabilia and the posterior of B. philadelphicus without noting 

 the slightest contraction, although Lemoine states that the 

 similar ones of B. parasita are contractile; but as he includes 

 the efferent tubules (which have no muscular coat in our 

 species) in this statement, the observation may require verifica- 

 tion. One would, however, on a priori grounds expect the 

 vesicles to be contractile. In all cases the lumen of the efferent 

 duct is much constricted at the point where it ceases to be 

 intracellular and becomes intercellular, i.e., where the walls of 

 the drain-pipe cells become continuous with the vesical epi- 

 thelium; and here a valve-like outgrowth guards the aperture. 



In B. philadelphicus the common vesicle of the anterior pair 

 of nephridia is tubular, and hangs vertically from the dorsum of 

 the third somite into the spacious intermuscular space (Figs. 

 44-46), reaching almost to the inner layer of longitudinal 

 muscle fibres, where the two efferent ducts open close together. 



