. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. 10 NERVES OF MOLLUSKS. were, two parallel series. The series of articulate animals seems to be the highest in the scale; but the raollusca, though less per- fect in structure, seem to be less remote in their mode of orga- nization from the vertebrata, and for this reason we have deter- mined to place them first. 3. We give the name of Mollusca (from the Latin, mollis, soft) to animals which, in the general features of their orga- nization, resemble snails, oysters, &c. They have no articu- lated skeleton nor
. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. 10 NERVES OF MOLLUSKS. were, two parallel series. The series of articulate animals seems to be the highest in the scale; but the raollusca, though less per- fect in structure, seem to be less remote in their mode of orga- nization from the vertebrata, and for this reason we have deter- mined to place them first. 3. We give the name of Mollusca (from the Latin, mollis, soft) to animals which, in the general features of their orga- nization, resemble snails, oysters, &c. They have no articu- lated skeleton nor vertebral canal, like the vertebrate animals ; their body is soft, and their skin, which is often covered by a shell, is never hardened so as to form a kind of external skele- ton composed of a series of rings, as is the case in insects and the Crustacea. 4. The nervous system, an apparatus of so much importance, the action of which regulates all the phenomena of animal life, and the functions of which must necessarily be in harmony with its mode of conformation, differs altogether in the mollusca in its general disposition, from what we observe in the vertebrata. There exists a nervous centre which to a certain extent may be compared to the brain in superior animals; but this ganglionic mass is not continuous with an organ analogous to the spinal marrow ; the other central parts of the nervous system are found on the opposite side of the digestive tube, and hence this canal is always embraced by a sort of medullary collar, from which the different nerves of the body arise. In other respects, this appa- ratus varies more in its arrangement than in the number of parts composing b a Fig. 1. NERVOUS SYSTEM OF A SOLEN. 5. Tn a great many of the least perfect mollusks, such as the Solen and many other acephala, which inhabit bivalve shells, the nervous system is generally composed of but two pairs of ganglia, united by two long inter-ganglionic cords, giving rise to different
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectmollusks, bookyear184
