MOLLUSCA — CONCHIFERA. 763 



shell, enable the animal to open or shut it at pleasure. 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. 

 In some families, this sense appears to reside in tentacular filaments, w^hich 

 border the lobes of the mantle, or certain places of these lobes. These 

 tentacular threads, which appear very sensible, or at least irritable, are, in 

 general, numerous, short, very fine, and move sometimes with extreme 

 quickness. The heart in the Conchifera is placed towards the back. It is 

 small, but provided with venous and arterial vessels. The liver is large, 

 embracing the stomach and a great portion of the alimentary canal. The 

 bronchiae are external, and appear more particularly so in those in which the 

 mantle is open before. These bronchiae 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 bronchiae are formed of a tissue of small 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 teeth, 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 eqinvalve. 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, it is then tense, the valves being held together by the con- 

 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 on a 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 Avitlulraws 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, 



