SEA-FISHERIES LABORATORY. 447 
below its upper extremity gives oft a tube which is lined 
by columnar ciliated cells, differing 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 45):—-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 surface 
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 1s much reduced and divided 
up by these folds, longitudinal folds predominating 
(fie. 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 by the sections (fig. 43); and so in this way the 
glandular area is increased very much, and the cavity 
broken up and reduced in size. ‘Two layers are concerned 
in the formation of these folds, the connective tissue 
(ies 42, RH. 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. 
