990 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
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 BRANCHIZ. 
The general arrangement of the branchize (ctenidia) with 
respect to the other parts of the body, has already been 
described above. There is a single ctenidium on each 
side. Hach 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 
lamelle. The lamelle of the same branchia are bound 
together, but there is no connection between the two 
branchize except at the base of the ctenidium from which 
both take origin. 
The base of the ctenidium (B7"., fig. 3, Pl. L.) 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 
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