308 ELEMENTARY BIOLOGY. [CHAP. 



The currents are produced and kept up by the action of 

 the cilia, which abound upon the gills, labial palps and 

 inner face of the mantle. The gills are perforated by in- 

 numerable small apertures, and the cavities contained 

 between the two lamelbe of which each is formed, are 

 in communication, above, with the cloacal chamber. The 

 cilia work in such a way as to drive the water in which the 

 animal lives from the outer surface of each gill towards its 

 interior. Hence, to a large extent, the current which sets 

 from the infra-branchial to the cloacal chamber. 



The current of water which is thus continually drawn into 

 the infra- branchial chamber carries with it minute organ- 

 isms, Infusoria, Diatoms and the like, and many of these 

 are swept to the fore part of the chamber, where they enter 

 the mouth, and are propelled by the cilia which line its 

 cavity into the alimentary canal. The latter presents a 

 short and wide gullet, a stomach surrounded by a digestive 

 gland, a long intestine coiled upon itself, in a somewhat 

 complicated manner, and, finally, a rectum, which lies in 

 the middle line of the dorsal aspect of the body, traverses 

 the pericardium and the heart which lies therein, and ends 

 in the anus. 



The subdivision of the mantle-cavity into supra and 

 infra branchial chambers is, in the long run, the result of 

 confluence of adjacent gill-lamina? with each other and 

 with the pallial lobes. The gill-perforations, the groove en- 

 closed by the labial palps and the aperture of the mouth 

 towards which it leads, all open into the lower or inha- 

 lent chamber; while the upper or exhalent one receives 

 the anus, and both genital and excretory orifices. During 

 life the thickened margins of the pallial lobes are in close 

 apposition ; consequently, the infra-branchial chamber is 

 closed below and in front, both it and the supra-branchial 



