TRANSVEESE SECTIONS. 105 



above the visceral mass and opposite the bases of 

 the inner gills. 



If the section has passed a little in front of 

 the excretory apertures, the reno- pericardial pas- 

 sages from the pericardial cavity to the anterior 

 ends of the kidneys are well seen. 

 b. The ureters are cut through at the part where they 

 communicate with each other across the median 

 plane : they appear as a wide cavity immediately 

 below the pericardium and above the kidneys. 

 The external openings are to the outer sides of 

 the kidneys, between the two lamellfe of each 

 inner gill. 

 7. The reproductive apertures are immediately ventral to 

 the excretory apertures ; they may be followed with 

 a seeker into ducts leading from the genital gland. 



B. Transverse Section through the Middle of the Ventricle. 



1. The mantle-lobes have the same appearance as in the 



previous section. 



2. The visceral mass is cut across close to its posterior end, 



and hence is much smaller than in the previous sec- 

 tion ; it is occupied almost entirely by the ovary, or 

 testis. 



3. The alimentary canal. The intestine is cut across twice 



in the visceral mass ; and the rectum with the 

 typhlosole is cut across as it lies in the pericardial 

 cavity. 



4. The gills. The attachments of the gills are the same 



as in the previous section, except that the inner 

 lamella of the inner gill ends in a free edge above. 



At the base of each gill, between its lamellsB, is a 

 wide supra-branchial passage. The inner canal of 

 each side communicates with the branchial cavity 

 through the slit between the unattached inner lamella 

 of the gill and the base of the visceral mass. 



5. The circulatory system. 



