NERVOUS SYSTEM. 181 



and this portion of the nervous system, appears to be com- 

 posed only of two unequal nervous masses. In the Cepha- 

 lopods the coalescence of the ganglions is more intimate, so 

 that they now form a continuous circle of nervous matter 

 around the gullet, enclosed in a cavity in the posterior part 

 of the cartilage of the head — in, in fact, a sort of rudi- 

 mentary skull. Thus you observe, that the nervous system 

 advances in a regularly proportional degree with the com- 

 plexity of the general organization, and especially with the 

 muscular system.* 



From the various ganglions nerves depart to carry the 

 influence of their centres to the different organs of sense, 

 and to the various viscera. These filaments are usually 

 simple. In many of the higher groups of mollusks some of 

 them trend to and unite with other ganglions, thus more 

 intimately connecting distant parts together. These se- 

 condary ganglions vary in position with the varying modifi- 

 cations of the whole structure, and are unsymmetrical ; and 

 this want of bilateral symmetry has led Professor Owen 

 to denominate the class or sub-kingdom, Heterogangliata.f 

 The nerves themselves cannot be resolved into smaller 

 filaments, like those of vertebrate animals ; they are formed 

 apparently of a soft homogeneous medullary matter, sur- 

 rounded with a sheath so loosely adherent thereto that it 

 can be filled with injections ; whence some have been led to 

 suppose that the nerves are hollow, and others that the 

 tunics are the vessels of the lymphatic system. The colour of 

 the ganglions in some mollusks is remarkable. Cuvier found 

 them bright red in the Limneus stagnalis and Planorbis cor- 



* This slight sketch, sufficient for our purpose, is derived from Milne- 

 Edwards Ele'mens de Zoologie, 241 — 244. The English student has excel- 

 lent resources to extend his knowledge of the subject in the elementary 

 works of Professors Grant, Owen, and Jones. 



t "In the mollusca, the nervous system is principally concentrated around 

 the entrance to the alimentary canal, forming a circle of ganglia through 

 which the oesophagus passes, and which is connected with other ganglia, dis- 

 posed without symmetry among the viscera, or in the neighbourhood of the 

 organs of locomotion, if such should be specially evolved. In some of the 

 highest of this division, the nervous system approaches very closely in its ar- 

 rangement to the form it presents in the lowest vertebrata, and receives a 

 corresponding protection by a rudimentary internal skeleton ; but, in general, 

 it is more connected with the immediate supply of the nutritive functions, 

 and wants that symmetrical arrangement and close connection with the 

 locomotive organs which may be regarded as characters of elevation in the 

 nervous system of the Articulata. From the general plan of the distribution 

 of their ganglia, mollusca have been termed cyclo-gangliata." — Carpenter's 

 Gen. and Comp. Physiology, p. 71. The latter name is that bestowed on 

 the class by Professor Grant. 



