34 



THE POPULAR SCIENCE MONTHLY. 



there are (Fig. 11) no supra-oesophageal ganglia. There is nothing, 

 in fact, to which the term brain can be appropriately applied. 



But, if we turn now to the much more active snail, we find the ner- 

 vous system existing in a more developed and concentrated form. 

 There is (Fig. 12, I) a large ganglionic mass situated over the oesopha- 

 gus, each half of which receives a considerable bundle of nerve-fibres 

 (f) from the eye (b) of the smaller side, which is situated at the tip of 

 the larger tentacle. It also receives another bundle of nerves (k) 



Fia 12. Head and Nekvous System of the Common Snail. 



from the small tentacle on each side, which has in all probability a 

 tactile function. The auditory vesicles are here in a new position. 

 They are in immediate relation with the posterior aspect of these 

 ganglia constituting the brain, though in other gasteropods they are, 

 as in bivalve Mollusca, found to be connected with the pedal ganglia. 

 That gasteropods are endowed with a rudimentary sense of smell is 

 now generally admitted by naturalists, though hitherto they have 

 been unable to locate this endowment in any particular organ or sur- 

 face-region. 



The brain of the snail is connected, by means of a triple cord or 

 commissure on each side of the cesophagus, with a still longer double 

 ganglionic mass (m). This latter body, situated beneath the oesopha- 

 gus, represents the pair of pedal and the pair of branchial ganglia of 

 the bivalve Mollusca. Here nerves are received from the integument 

 and given off to the muscles of the foot, while they are also received 

 and given off from the respiratory and other organs. 



In the nautilus and some other representatives of the next class, 

 Cephalopoda, the nervous system attains a development only slightly 

 in advance of that met with among the highest gasteropods, though 

 in the active and predaceous cuttle-fish, and in its near ally, the octo- 

 pus, we find the nervous system presenting the highest development 

 to be met with araons; the sub-kino-dom Mollusca. 



