EXTERNAL APERTURES 83 



adding a little colouring matter, will pass in and 

 out of the posterior end, the ingoing or inhalent 

 stream being ventral, the exhalent dorsal. 



This current is maintained by the cilia of the 

 mantle, gills, and palps : it serves to bring water 

 for nutrition and respiration, and to carry away 

 the fascal matter and the excretory and reproduc- 

 tive products. 



i. The inhalent aperture, through which the in- 

 halent stream enters the branchial chamber, 

 is a vertical slit at the hinder end of the 

 body : it is bordered laterally by the thickened 

 posterior edges of the mantle-lobes, which 

 bear the tentacular fringes. Below, it is in- 

 completely closed by the approximation of the 

 edges of the mantle-folds; and above, it is 

 separated from the exhalent aperture by a 

 horizontal partition formed by the fusion of 

 the gills of the two sides with each other, 

 ii. The exhalent or cloacal aperture is much smaller 

 than the inhalent opening, and lies imme- 

 diately dorsal to it, and behind the posterior 

 adductor muscle. It is bounded laterally by 

 the thickened mantle -borders, which have 

 here no tentacles. 



Pass a seeker into the cloacal opening, and forwards along 

 the supra-branchial cavity above the gills. Note the partition 

 between the exhalent and inhalent apertures. 



c. The anus opens into the cloacal cavity, close to its 



external opening. 



Insert a seeker into the groove between the margins of the 

 mantle-lobes above the posterior adductor, and pass it back 

 over the adductor into the cloacal cavity. Slit up this canal 

 so as to expose thoroughly the hinder part of the rectum and 

 the anus. 



Insert a seeker into the anus and pass it upwards and 

 forwards along the rectum as it lies on the dorsal surface of 

 the posterior adductor. 



