3372 



THE MOLLUSKS OR SHELLFISH 



and other substances; its form consequently being very variable. The mouth (b) 

 is situated at the anterior end of the body, at the upper front part of the foot, form- 

 ing a simpletransverse aperture. The lips, upper and lower, are usually pro- 

 longed on each side into two lobes (c), or labial palpi. These vary in form, but 

 are mostly triangular. In some groups they are very large (Tellinide) t but in 

 others they are practically wanting. The mouth is not armed with jaws or 

 radula, for in creatures which never prey upon other animals, or go about 

 seeking their food, such structures would be useless. Bivalves obtain their food 

 consisting of microscopic organisms in the course of respiration. Whatever 

 is carried in by the inflowing current is collected on the gills, and then con- 

 veyed by the palpi to the mouth. The leaf-like gills (d, e] are arranged on each 

 side of the body, and inclosed by the mantle. Each gill is partly attached by 

 its upper or dorsal margin. This gives rise to two rows of hollow filaments, 

 which are in a few cases simple and disposed in opposite directions, but are gener- 

 ally parallel with one another and directed toward the ventral side, with the fila- 

 ments long and refolded upon themselves, so that each row forms a double lamella. 

 These filaments are united one to another by cilia. The gills and the inner surface 



of the mantle cavity are covered with 

 microscopic cilia, which through their 

 vibratile motion produce the currents of 

 water necessary for respiration. The water 

 generally flows into the pallial cavity at the 

 posterior ventral side, and there is filtered 

 through the gills, passing out again poste- 

 riorly through the anal opening. The nerv- 

 ous system consists of three pairs of ganglia, 

 a cerebral or supraoesophageal, a pedal, and 

 a visceral pair. The cerebral ganglia (i) 

 are mostly placed above the oesophagus, the 

 pedal pair (2), as their name implies, within 

 the foot, and the last pair (3) are situated in 

 front of or just beneath the posterior adduc- 

 tor muscle. 



Most pelecypods are endowed with the 

 senses of touch, smell, and hearing, and some 

 are provided with eyes. These are found 

 either upon the edges of the mantle or at 

 the end of the siphons; and in some forms, 



such as Spondylus and Pecten, they are highly developed. The sexes are generally 

 distinct, but occasionally united. The young are produced from eggs, which are 

 either cast free in the water, or are hatched between the branchial lamellae of the 

 parent. The shell, as already stated, is composed of two pieces, right and left valves, 

 which are protective of the soft parts, and correspond each to a lobe of the mantle. 

 They are generally of equal size and shape, but in certain groups they differ con- 

 siderably in these respects. They are nearly always joined dorsally by an elastic 



ANATOMY OF RIVER MUSSEI,. 



