PATELLA. 205 



almost completely encircle tlic visceral hump except at 

 the extreme left front. The visible parts of the kidneys 

 lie very superficially, closely bound to the visceral 

 integument, and their boundaries can, as a rule, only be 

 clearly seen in injected specimens, since, in uninfected 

 ones, the digestive gland often shows through by trans- 

 parency. As the renal waste is dark coloured, the kidney 

 is fairly often naturally tinted so as to make its outline clear. 

 III. Nuchal Cavity and Related Apertures. — The 

 continuous mantle cavity which lodges the pallial gills 

 deepens in front to form a fair sized nuchal cavity, so- 

 called because it lies above what may be called the neck 

 region of the animal. It opens by a wide slit between the 

 anterior ends of the shell muscle. Its roof is formed by 

 a backward extension of the mantle, and its floor by the 

 muscular body wall of the neck reg'ion. With the 

 exception of the mouth, all the apertures of the body 

 open into it, i.e., the anus, the left renal opening, and 

 the right renal opening (figs. 4 and 5). When the cavity 

 is opened by careful removal of its roof the following 

 structures can be made out : — 



(a) The anal papilla opening into the back of the 

 cavity upon the animars right shoulder. 



(b) On either side of this a renal papilla, the right 

 being rather more conspicuous than the left. 



(c) The pericardium, which bulges into the cavity along 

 a line from the left tip of the shell muscle nearly to the 

 left renal papilla. 



(d) The pallial vein (also visible from outside before 

 opening the nuchal cavity) at the left side of the cavity, 

 running into the pericardium. 



(e) The small " pulmonary veins " running in from the 

 roof of the nuchal cavity (visible before opening) and 

 piercing the pericardial wall to the right of d. 



