CEPHALOPODA. 437 



The excretory organs lie on the posterior side of the visceral sac 

 close to the mantle-cavity, into which they open. In Dibranchs they 

 are two in number and are either completely separate (Octopoda), 

 or they are connected together, generally through an anteriorly and 

 dorsally-placed portion which is of considerable extent and lies close 

 under the shell. This unpaired portion contains the so-called pan- 

 creatic tissue of the bile ducts, and is in relation with these 

 structures. The anterior and posterior afferent branchial vessels 

 run in the Avails of these kidney sacs on their way to the gills, 

 and give off blind diverticula which project into the cavities of the 

 kidneys. The kidney epithelium, which is flattened over the rest 

 of the sac, is especially glandular on these vascular diverticula 

 (Fig. 349, XII) and secretes the waste matter in the form of 

 concretions which largely consist of guanin. 



Each kidney opens into the mantle-cavity through a papilla placed 

 not far from the anus, and into the pericardial coelom by a pore not 

 far from the external opening. 



In Nautilus there are four kidneys which open to the exterior; 

 one pair opens just anterior (i.e., ventral) to the anterior gills, and 

 the other pair just anterior to the posterior gills and close to the 

 openings of the pericardium ; they are without any internal opening 

 into the coelom. The four afferent branchial vessels run in the wall 

 separating the four kidneys from the pericardium, and give oft' 

 diverticula, which are covered with glandular tissue, into the peri- 

 cardium as well as into the renal sacs. The four tufts of glandular 

 processes thus projecting into the pericardium constitute the peri- 

 cardial gland, and are larger than the corresponding processes on the 

 opposite side of the septa projecting into the renal sacs. 



The kidneys and gills of Nautilus have been spoken of in the text as anterior 

 and posterior, but it must be borne in mind that, owing to the fact that the 

 part of the body in which they lie has been prolonged vcntrally in the mantle- 

 fold, the posterior kidney and gill are really ventral, i.e., nearer to the mouth 

 than the anterior, which are placed just at the point where the mantle-fold 

 is given off from the body. The pericardium, which is also in the mantle- 

 fold, is actually posterior to the kidneys, and the posterior walls of the 

 kidney-sacs form the anterior wall of the pericardium ; but it must be 

 remembered that morphologically the pericardium is dorsal to, i.e., nearer the 

 apex of the visceral sac than, the kidneys. 



The Cephalopoda are dioecious. The sexes present external differ- 

 ences, which are sometimes very marked. In many cases the males 

 are much smaller than the females, as in Argonauta, in which genus 



