SEA-FISHERIES LABORATORY. 919 
sac and the outer wall is the blood sinus surrounding the 
secretory tubules. The renal sinus communicates with 
the pedal sinus by perforations in the muscular wall of 
the.viscero-pedal mass, as the pedal retractors originate 
from the latter. The muscles he actually in the blood 
sinus. In front of them the sinus is a wide central 
cavity lying beneath the flattened renal sac, with renal 
tubules dipping into it on all sides except dorsally. 
The body-wall is gathered up into a pair of lateral folds, 
which take part in the formation of the bases of the 
etenidia, and the blood from the central cavity filters 
through the spaces between the tubules into these 
lateral folds, and so into the gills. 
The reno-pericardial canals (fen.per., fig. 4, Pl. IL.) are 
a pair of large tubules which take origin on the lateral 
internal walls of the lateral posterior divisions of the renal 
sac. In sections they may be found on the part of the 
wall lying on the outside of each retractor muscle. They 
open into the renal sac by wide fimbriated mouths. Their 
walls near these openings are often peculiarly modified, 
and are produced out into several small diverticula, 
resembling the renal tubules. The canals pass down- 
wards and forwards along the ventral wall of the renal sac, 
and come to open into the pericardium on the floor of the 
latter underneath the ventricle, by a pair of prominent 
slits. All along their course the tubules carry a lining of 
long cilia, and on the openings of the canals into the peri- 
cardium these are very prominent, especially in young 
specimens (0°5 to 1 cm.), where they sometimes form a 
ciliated fringe projecting into the latter. 
The form and course of the renal tubules are very 
irregular, and in sections through tne posterior margin of 
the renal organ it is at first sight difficult to distinguish 
between the tubules themselves and the blood sinus. This 
