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 (Tellinidcii), 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 supracesophageal, 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 lamellm 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 
