332 CHARACTERS OF INVERTEBRATE ANIMALS 



the labial palps, may be seen on either side of the mouth. The 

 body hangs down between the mantle-lobes, and its ventral part 

 is modified into the orange-coloured foot, which, instead of pre- 

 senting a creeping under surface, as in a snail, is flattened from 

 side to side and projects forwards as a muscular body which has 

 been variously compared, as regards its shape, to an axe or 

 ploughshare. It can be protruded from between the valves of 

 the shell, and serves as a pushing-organ, by which the animal can 

 slowly plough its way through the mud with its front end first. 



Very conspicuous are the plate-like gills, which have suggested 

 the scientific name of the class (Lat. lamella, a plate ; Gk. branchta, 

 gills), and which are not only breathing-organs but are also largely 

 concerned with setting up the currents of water which play such 

 an important part in the life of the animal, being largely aided, 

 however, in both these duties by the lobes of the mantle. The 

 water-currents are a result of ciliary action (see p. 49). Each 

 gill consists of an outer and an inner plate, and, despite its special- 

 ized form, has been produced by the modification of a gill-plume 

 similar in kind to those found in the Ormer (see p. 308). The 

 stem of the gill runs fairly parallel to the long axis of the body, 

 and is attached to the body- wall above. The mantle - cavity is 

 here the huge space between the mantle-lobes into which the 

 gills and lower part of the body hang down, and, by the attach- 

 ment of the former to adjacent parts, it is divided into a large 

 lower section into which the inhalent aperture leads and a much 

 narrower upper portion, above the gills, and communicating with 

 the exterior by the exhalent aperture. This is most clearly seen 

 behind the posterior adductor muscle, where the inner plate of 

 one gill is seen to be fused along the middle line with the corre- 

 sponding plate of the other gill, thus forming a partition between 

 the upper and lower sections of the mantle-cavity. 



Digestive Organs (fig. 192). — The most striking feature is a 

 negative one, consisting in the entire absence of the characteristic 

 rasping organ (odontophore) possessed by the other moUuscan 

 classes. It is believed that bivalves are descended from ancestors 

 which were provided with this structure, which has been lost as 

 a result of the same conditions which led to the dwindling of 

 the head, and which have already been alluded to. The mouth 

 leads into a short gullet, which opens into a stomach, that again 

 continues into a coiled intestine, the last part of which runs up 



