180 NERVOUS SYSTEM. 



and which vary in number and in perfection in the different 

 tribes. Our account of these it may be convenient to pre- 

 face with a very short and general sketch of the nervous 

 system, as from it emanate all their powers. 



In the compound Tunicata the existence of a nervous sys- 

 tem is doubtful ; and in the Ascidians it is only slightly 

 sketched. Here its chief centre or ganglion is situated " in 

 the interspace between the two openings of the muscular 

 tunic," and from this centre branches are sent to each aper- 

 ture, to the respiratory sac, and to the digestive organs. In 

 the Brachiopods the system is not more developed than in the 

 Ascidians, and in some genera is very obscurely adumbrated. 

 In the proper mollusca the system consists of several ganglions, 

 always paired and associated together by filaments or nerves. 

 One pair, usually called cerebral and considered analogous 

 to the brain of vertebrates,* is situated on the dorsal side 

 of the body in front and above the gullet, and is connected 

 by two filaments to a pair of abdominal ganglions placed fur- 

 ther backwards and on the gullet's opposite side, which is 

 thus encircled with a collar of double nerves. A third pair 

 of ganglions, in general less developed than the others, is 

 found under the anterior extremity of the oesophagus, and 

 communicates with the cerebral by two filaments that form 

 a circle similar to the one already mentioned. These are the 

 labial ganglions. They are absent in some genera, while, in 

 others of the same order, additional ganglia even are found $ 

 but the differences in the complexity of the nervous system 

 depend ordinarily on the degree of developement of the 

 centres already specified, and in their more or less nighness 

 to each other. Thus in some mollusca low in the scale, as 

 in the Razor- shell (Solen), the cerebral ganglions are very 

 widely set apart ; yet even these are always united by a 

 commissure, and the abdominal ganglions are placed at the 

 opposite extremity of the body, so that the filaments 

 representing the oesophageal collar are of comparatively 

 excessive length. Thus also in the Ianthina, the four oeso- 

 phageal ganglions are still distinctly isolated, but the poste- 

 rior pair are so closely approximated to the cerebral that 

 the connecting commissure encircles the oesophagus with a 

 strict embrace. In the snail, and in a crowd of others, the 

 centralization is carried further, for both the cerebral gang- 

 lions and the sub-cesophageal touch on the mesial line, 



* Improperly, I believe. " Les nerfs qui viennent du cerveau se distri- 

 buent plutot comme ceaux de la vie organique, ou le grand sympathique, 

 que comme les nerfs cerebro-spinaux des vertebres." — Cuvier, Hist. Sc. 

 Nat. iii. 60. M. Serres denies that the mollusca have a brain. 



