No. 3-] THE DISCODRILID NEPHRIDIUM. 355 



in fact conjointly, into its lower end. It is lined, except at the 

 extreme lower end, by the invaginated epidermis and cuticle 

 (Figs. 44-46, ve), and is clearly surrounded by a thin mus- 

 cular ivm) as well as by an outer connective-tissue {ct) coat. 

 The position of the muscular layer, in close contact with the 

 epithelium, indicates that it originates from the circular layer, 

 but it is noticeable that the vesicle is pressed upon by the fibres 

 of the outer longitudinal layer, between two of which it passes 

 (Fig. 44, hne). A number of small fibres are seemingly dis- 

 placed from this layer and lie more deeply, in contact with the 

 sides of the vesicle (Fig. 44, ;;/). 



The anterior vesicle reaches its highest development in B. 

 illuminatus, in which, owing to the shallowness of the inter- 

 muscular space, it is forced into a horizontal position (Fig. 47), 

 and causes the longitudinal muscles to bulge slightly into the 

 body cavity. Thus its greatest length is directed antero- 

 posteriorly instead of vertically; and instead of being tubular 

 it becomes flattened-gourd-shaped, with a somewhat narrowed 

 neck, as seen in longitudinal section (Fig. 51). Its pore is 

 situated on a rather distinct papilla having the same situation 

 as in B. philadelphicus. The general structure is the same as 

 in the latter species, but owing to the much larger size of the 

 epidermal cells, the lining epithelium is thicker (Figs. 50, 51), 

 and the number of cells fewer; and moreover they at one point 

 extend into the lumen in such a way as to form an internal 

 diaphragm, which imperfectly divides the interior into two cham- 

 bers communicating through a central opening (Figs. 47, 48, 

 51). The anterior and larger compartment {e) opens to the 

 exterior, and is alone lined by cuticle. The inner one receives 

 the efferent tubules, which in entering the vesicle become first 

 attached to its walls at an anterior point, where the muscular 

 coat is perforated. They then bend sharply backward (Figs. 

 47, 48), the tubule walls passing into the epithelium, and the 

 lumen extending between the cells to enter the posterior or 

 inner chamber, one on either side {ec). In this species the mus- 

 cular layer (Figs. 50, vm, 51, m) is well marked, and is derived 

 mainly from the circular fibres, though the longitudinal are 

 closely related to its ventral wall. Of all species the vesicle of 



