M O L L U S C A. 



601 



MoiUucs- on the direct exposure to the light, as some have ima- 

 '"'"Y " gined, for many shells which are destitute of a cuticle 

 are white, while many of those covered with a dense 

 cuticle are finely variegated beneath. The forms of the 

 British species of shells have been described in detail 

 under the article CONCHOLOOY, where an explanation 

 of the terms by which their different parts are distin- 

 guished may likewise be found. 



Between the skin and the shell neither vessels nor 

 nerve* have been traced ; and the manner in which the 

 latter is formed forbids us to expect their existence. Yet 

 the shell cannot be considered as dead matter, so long as 

 it remains in connection with the living animal. In those 

 animals in which the shell is external, there are muscles 

 which connect the animal with its internal surface, and 

 the bond of union being a substance soluble in water, 

 the muscle can be detached by maceration. The analogy 

 between shell and bone is here obvious, although in the 

 one case the connection between the muscle and the 

 bone is permanent, in the other between the muscle 

 and shell temporary, or frequently changed during the 

 life of the animal. But the vitality of the shell, if I 

 may use the expression, is demonstrated, from the chang- 

 es which it undergoes when detached from the animal : 

 The plates of animal matter harden ; the epidermis dries, 

 cracks, and falls off; and in many cases the colours 

 fade or disappear. We confess ourselves unable to 

 point out the means employed by the animal to prevent 

 these changes from taking place, by any process similar 

 to circulation. It is probably effected by the secretions 

 of the skin, in the same manner as our cuticle and hair 

 are lubricated. When the shelly covering consists of 

 two or more pieces, they are joined together as the ar- 

 ticulated bone* in the higher classes of animals, by li- 

 gaments. These, in tome cases, are of great thickness 

 and strength, and, in consequence of their elasticity, 

 assist in the motion of the different part*. 



In the molluscous animals the skin secretes a vis- 

 cous, adhesive substance, differing according to the 

 medium in which the animal resides, but in all cases 

 calculated to resist its influence. It is probably owing 

 to the lubricating agency of this secretion that both the 

 cuticle and shell are preserved from decomposition. 



The skin likewise secretes the colouring matter by 

 which the shells arc variegated. The glands from 

 which it proceeds vary much in different individuals, 

 and even in the same individual in different periods of 

 growth. 



The characters furnished by the skin and its appcn. 

 dices are extensively employed in the systematical ar- 

 rangement of molluscous animals ; nearly all those cha- 

 racters which distinguish the species, and many of 

 those on which genera are established, are derived from 

 the form of the shell, the tentacula, or the colour. This 

 last character, however, is one on which little depend- 

 ence should be placed. 



Mwrulsr 3. Mtucvlar System. There is nothing peculiar in 

 ijttetn. the muscular system of this class of animals. Where 

 the muscles are inserted in the skin, as is usually the 

 case, that organ is in some cases strengthened by con- 

 densed cellular substance ; where the muscles are in- 

 serted on the shell, although no intervening tendons 

 can be perceived, the existence of a connecting link of 

 a similar nature is rendered probable by the circum- 

 stance of boiling water not dissolving the muscle itself, 

 yet detaching it from the surface of the shell. During 

 the growth of the animal, such detachment is frequent- 

 ly performed voluntarily as it increases in size, and al- 

 ters its position with regard to the shell, 

 tot. xiv. PART n. 



Molluscous animals preserve themselves in a state of Mollusca. 

 rest chiefly by suction and cementation. The organ i ' 

 which acts as a sucker, is in some cases simple, soft, 

 and muscular, as the foot of the snail, while in others 

 it is compound, and strengthened internally by hard 

 parts as in the arms of the cuttle fish. The force with 

 which some animals adhere is very considerable, and is 

 strikingly displayed, for example, when we attempt to 

 detach a limpet from the rock. 



The rest which is maintained by cementation, in 

 some cases depends on a glairy secretion, which glues 

 the body of the animal to the substance to which it is 

 disposed to be attached. By such an expedient the 

 shells of snails adhere to rocks, stones, and plants. It 

 is probable that the bivalve shells of the genus Cyclas, 

 which readily adhere to the sides of a glass, obtain their 

 temporary attachment by means of their glutinous cu- 

 ticle. In other animals threads are produced (termed 

 a Bysnts) from particular glands, and while one extre- 

 mity is glued to the rock, the other remains in connec- 

 tion with the animal. But there is an attachment more 

 durable than any of these, which takes place in some 

 shells, they being cemented to rocks or stones by calca- 

 reous matter, and retained in the same position during 

 the whole term of their existence. 



The locomotive powers of the mollusca arc confined 

 to creeping and swimming. The former action is per- 

 formed by alternate contraction and relaxation of the 

 foot, or muscular expansion, which serves as a sucker, 

 and is analogous to the motion of serpents. The mo- 

 tion of swimming is executed either by the serpentine 

 undulations of the foot and the body, or by the action 

 of tentacula, or expanded portions of the integuments. 

 Many species are aided in swimming by being able to 

 vary the specific gravity of their body at pleasure, and 

 either rise or sink in the water as circumstances may 

 require. In some, as the Janthina, there is a cellular 

 organ peculiarly destined for this purpose, which may 

 be regarded as in some measure anslagous to the air- 

 bladder of fishes. In all these exertions their progress 

 is proverbially slow. Some bivalve shells have the 

 power of leaping or shifting their position by a sudden 

 jerk, produced by shutting the valves rapidly. This 

 is strikingly displayed in the common Scallop, and is 

 less perfectly exhibited in the river mussels. In a tew 

 instances, especially among the slugs, a thread is form- 

 ed of the viscous secretion of the skin, by which the 

 animal is enabled to suspend itself in the air from the 

 branches of trees. 



Although the progressive motions of molluscous ani- 

 mals are 

 tions ; 

 bility 



go great, that the protecting movements are executed al- 

 most instantaneously, and the organs are contracted or 

 withdrawn into the body. But these rapid exertions 

 are only called forth in the moments of danger ; the or- 

 dinary movements are all executed with characteristic 

 slowness. 



The characters furnished by the muscular system, are 

 of great value in the discrimination of species, and in 

 the construction of genera and higher divisions. They 

 are intimately connected with the habits of the animal, 

 and merit the attentive examination of the philosophi- 

 cal naturalist. 



4. AVrvoiu Sytlem. In the molluscous animals the K 

 nervous system is less complicated in its structure than oui 

 in the higher classes, and the brain is not restricted in **' 

 its position to the head. The whole nervous system 

 4o 



Aitnougn me progressive motions ot molluscous ani- 

 mals are comparatively slow, the other muscular ac- 

 tions are executed with ordinary rapidity. The irrita- 

 bility of some parts, as the tentacula and branchiae, is 



