402 



MOLLUSCS. 



lips, upper and lower, are usually prolonged 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 (Tellinidce), 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 

 conveyed by the palpi to the mouth. The leaf-like gills (d, e) are arranged on 

 each side of the body, and enclosed 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 

 generally parallel with one another and directed towards the ventral side, with the 

 filaments 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 

 posteriorly through the anal opening. The 

 nervous system consists of three pairs of 

 ganglia, a cerebral or supraoesophageal, a 

 pedal, and a visceral pair. The cerebral 

 ganglia (1) 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 adductor 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 

 ligament, or resilium, and often interlocked at the same place by projections on 

 the edges of the valves, termed hinge-teeth. In the majority of species the valves 

 shut closely together all round the edges, but in many they gape at one or both ends, 

 or at the ventral side. In certain species of Pinna the two valves are actually 



ANATOMY OF 1UVER-MUSSEL. 



