106 ELEMENTARY BIOLOGY. [XL 



front of the mouth; at the sides, they are united with the 

 dorsal edges of the outer gill-plates ; and, behind, they extend 

 upwards and on to the dorsal face of the body, before finally 

 passing into one another above, and in front of, the anus, 

 which is small, tubular, prominent and median. Thus the 

 anus is inclosed in a part of the cavity bounded by the two 

 mantle lobes, which is relatively small and shallow, and is 

 termed the cloacal chamber ; while the gills, the foot, and the 

 palps, hang down into the relatively large branchial chamber, 

 which occupies the space between the mantle-lobes for the 

 rest of their extent. It is the prolongation of the margins 

 of the former cavity which gives rise to the tubular anal 

 siphon seen in so many Lamellibranchs ; while the ventral 

 or branchial siphon is a similar prolongation of the margins 

 of the branchial chamber. The dorsal siphon is the channel 

 through which the exhalent currents pass ; the ventral, that 

 for the inhalent currents. 



The currents are produced and kept up by the action of 

 the cilia which abound upon the gills. The latter are per- 

 forated by innumerable small apertures, and the chambers 

 contained between the two lamellae of which each gill 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 the current which sets 

 from the branchial to the cloacal chamber. 



The current of water which is thus continually drawn 

 into the branchial chamber carries with it minute organisms, 

 Infusoria, Diatoms and the like, and many of these are swept 

 to the fore part of the branchial 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 hepatic follicles, a long 

 intestine coiled upon itself, in a somewhat complicated 



