helow its upper extremily gives olt' a tube which is lined 

 by columnar ciliated cells, dittering much in appearance 

 from the glandular cells of the rest of the renal organ. 

 This reno-pericardial canal passes tO' the dorsal side of the 

 efferent branchial vessel, and opens into the pericardium 

 above it and rather to the outer side. Both renal organs 

 communicate at their upper ends by a transverse branch 

 running under the visceral mass above the adductor 

 muscle. The cells lining this channel have the same 

 appearance as those lining the rest of the cavity, but the 

 walls are not folded and the space between them is but small. 

 The structure of the renal gland is as follows (figs. 42 

 and 43): — Lying underneath the outer epithelium 

 (fig. 43, Ren. ep.) there is a connective tissue layer which 

 forms a definite sheath and supports the internal 

 glandular layer. Longitudinal and circular muscle fibres 

 occur in this connective tissue sheath. The inner siirface 

 of the gland, bounding the cavity, is complicated in its 

 folding. This can be seen by slitting up the side of the 

 renal organ, but better still by cutting transverse 

 sections. The whole cavity is much reduced and divided 

 up by these folds, longitudinal folds predominating 

 (fig. 6), but they are not regular and appear to bifurcate 

 and anastomose. The folds that are seen with the naked 

 eye are really themselves made up of numerous folds, as 

 shown b}^ the sections (fig. 43); and so in this way the 

 glandular area is increased very much, and the cavicy 

 broken up and reduced in size. Two layers are concerned 

 in the formation of these folds, the connective tissue 

 (fig. 42, 7?. con.) and the glandular layer (Ren. c). 

 From the prominent connective tissue sheath which 

 surrounds the organ, folds are given off which support the 

 epithelium. Thus, both layers are seen together forming 

 the folds. 



