76 



can be done, however, by the character of the wall ; the 

 internal surface of the renal tubule (fig. 9) is formed by an 

 irregular epithelium composed of large non-ciliated cubical 

 cells with clear or very faintly granular contents. The 

 cell walls are very definite ; the nuclei lie at the bases on 

 the lateral walls. The epithelium is supported on a 

 sharply defined basement membrane, from which bridges 

 of delicate fibrous tissue stretch across the blood spaces. 

 The wall of the renal sac, other than that lining the 

 tubule, has the same characters, except that the cells are 

 smaller and more irregular. 



The Branchi^. 



The general arrangement of the branchias (ctenidia) with 

 respect to the other parts of the body, has already been 

 described above. There is a single ctenidium on each 

 side. Each of these organs consists of two branchial 

 plates lying side by side in the mantle cavity, attached to 

 a base containing blood vessels. Each branchial plate or 

 branchia is again folded on itself so that it consists of two 

 lamelloe. The lamellse of the same branchia are bound 

 together, but there is no connection between the two 

 branchiae except at the base of the ctenidium from which 

 both take origin. 



The base of the ctenidium {Br' ., fig. 3, PI. I.) is a ridge of 

 the body-wall containing the blood vessels — the common 

 afferent and efferent branchial vessels. The afferent vessel 

 carries blood to both branchial plates, while the efferent 

 vessel carries away the blood oxidised by contact with the 

 water flowing through the mantle cavity. A flat band of 

 muscle fibres runs along the ctenidial base from the 

 posterior to the anterior extremities. The branchial 

 nerve terminates in the posterior portion of the base. 



The ctenidium of the left side is shown in fig. 2. The 



