MOLLUSCA—CONCHIFERA. 763 
shell, enable the animal to open or shut it atpleasure. The nervous system 
in this class is imperfectly developed, sensation very obtuse, and the brain, 
if such it may be termed, is a ganglion over the mouth, formed by the junc- 
tion of two nervous chords. ‘Their chief sense seems to be that of touch. 
Tn some families, this sense appears to reside in tentacular filaments, which 
border the lobes of the mantle, or certain places of these lobes. These 
tentacular threads, which appear very sensible, or at least irritable, are, :m 
general, numerous, short, very fine, and move sometimes with extreme 
quickness. The heart in the Conchifera is placed towards the back. It 1s 
small, but provided with venous and arterial vessels. The liver is large, 
embracing the stomach and a great portion of the alimentary canal. The 
bronchi are external, and appear more particularly so in those in which the 
mantle is open before. These bronchie are opposite, formed of large vascu- 
lar leaflets, generally crescent-shaped, placed on each side under the cloak, 
covering the belly of the animal, upon the sides of which they are attached 
in pairs. These bronchie are formed of a tissue of smal! vessels, arranged 
close together, like the pipes of an organ. At the sides of the mouth are 
four triangular thin leaflets, the extremities of two lips. All the Conchifera 
have a testaceous covering of two principal pieces, most of them of two 
alone. These pieces, named valves, are opposed to one another, and consti- 
tute the proper shell of the animal. The valves are united together near 
their base, by an elastic coriaceous or horny ligament, and the point of union 
is called the hinge. This hinge is distinguished by éeeth, or protuberances 
and hollows, which lock into each other when the shell is closed. When 
the valves are unequal or dissimilar in size, the shell is said to be inequi- 
valve ; and when, on the contrary, both resemble one another, in their 
general form and size, they are said to be eguivalve. Among the equivalve 
shells, however, are found some, which, when the shell is closed, have, 
towards their lateral extremities, an opening or gape, more or less consi- 
derable. In those in which this opening is large, it has been observed that 
the mantle of the animal is almost always united before. 
The ligament of the valves is sometimes exterior, and sometimes interior. 
In both cases, it serves not only to fix the two portions of the shell together, 
but to open them by its elasticity. When this ligament is exterior, if the 
shell be closed, itis then tense, the valves being held together by the cons 
traction of the internal muscle ; but if this muscle is relaxed, the elasticity 
of the ligament alone separates the valves. When, on the contrary, the 
ligament is interior, it is compressed when the shell is shut, and the muscle 
exerts its power, but throws open the valves when this power is relaxed. 
Though the Conchifera never crawl ona ventral disc, or foot, like many 
of the Mollusca, yet some possess a muscular, contractile organ, often com- 
pressed and lamelliform, which the animal exerts or withdraws at will. 
This muscular part serves some families as an organ of locomotion, by 
enabling them to execute a sort of leap; in others, deprived of locomotion 
