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 it. 



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 branches {Jig. 1); the ganglia of the anterior pair 



Explanation of Fig. 1. — Nervous system of an acephalous mollusk 

 (Solen) : — a, the pair of gang'lia which is situate in front of the cEsophagus 

 representing that portion of the nervous system of these animals, which is 



3. What are the general characters of the MoUus'ca ? 



4. What are the peculiarities of arrangement of the nerves of the Mollus'ca? 



5. What are the general characters of the nervous system in acephalous 

 mollusks ? What are tnc peculiarities of the nervous system of gasteropods? 



