5^6 Natural History of Molluscous Animals : — 



principle which directs the Pholades in their operations, which 

 moors the muscle to the rock, and to all others teaches them 

 their proper devices. 



The happiness of molluscous animals, then, depends on 

 the possession of life, and on the play of its functions ; and, if 

 thence we estimate their pleasures at a low scale, we must 

 remember that their pains and sufferings are proportionably 

 slight. Their days pass away in an even stream of quietness : 

 there is no anger to ruffle, no disappointments to sour them ; 

 they are amply provided by Him who careth for all, and they 

 take no care for to-morrow ; and, if it prove the precursor of 

 evil, the evil has been unforeseen and undreaded. But many of 

 this class of animals have additional means of enjoyment in the 

 organs of sense with which they are furnished, and which vary 

 in number and in perfection in the different tribes. Our 

 account of these it may be convenient to preface with a very 

 short and general sketch of the nervous system, as from it 

 emanate all their powers. 



In the Molliisca, this system consists of a central ganglion, 

 considered as being analogous to the brain, and placed above 

 the intestinal canal ; of a ganglion for each separate sense, 

 and for the organs of locomotion ; of some irregular visceral 

 ganglia ; and of nerves which connect the whole together, and 

 which permeate every part of the body. To adopt an illus- 

 tration from Cuvier, we may liken the system to a loose and 

 complicated network, in which the greater portion of the 

 threads communicate with each other, and in which there 

 appear, at different places, masses or enlargements, more or 

 less conspicuous, which may be considered as centres of com- 

 munication * : and the comparison is the more apt, as these 

 ganglionic enlargements are not masses of medullary matter, 

 but appear rather to be knots, formed by a compact and inti- 

 mate union of the filaments. The nerves themselves, how- 

 ever, cannot be resolved into smaller filaments, like those of 

 vertebrate animals ; they are formed apparently of a soft ho- 

 mogeneous medullary matter, surrounded with a sheath so 

 loosely adherent thereto that it can be filled by 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. f The colour of the ganglia in some Mol- 

 lusca is remarkable. Cuvier found them bright red in the 

 Lymnse^a stagnalis and Planorbis cornea; in the Aplysiae 

 they are blackish red and granular ; and Carus asserts that 



* Comp. Anat., trans., ii. 102. 



f Fleming's Phil. Zool., ii. 408. ; Edin. Phil. Journ., xi. 419. 



