HELTCID^B. 403 



in the largest spiral turn of the shell. In order to expel the air 

 respired by the lung, it withdraws its hody into the narrower part of 

 the shell, where it gathers itself up completely, even to its head and 

 feet, and hy this compression of all its little heing it expels the air 

 which fills it. These respiratory movements, however, are not regular, 

 but succeed each other only at certain intervals. Life would be too 

 hard for the poor snail were it passed in such violent efforts as would 

 be necessary if it respired as the larger animals do. In its case the 

 breathing is intermittent and imperfect ; it is merely a rough attempt, 

 as it were, at respiration, which becomes perfect in some of the higher 

 branches of the animal kingdom. 



The snail has a heart, consisting of a ventricle and auricle, con- 

 nected with a well-developed arterial vascular system, while the venous 

 system is imperfect. In short, the blood only returns from the various 

 parts of the body to the respiratory apparatus, after traversing lacunae, 

 or air-cells, existing between the several organs. 



The blood of the snail is of a pale rose colour, slightly tinted with 

 blue. It has a rudimentary brain, composed of a pair of thick 

 ganglions, situated above the oesophagus, which are in connection 

 with another pair of ganglions placed below, which, together, form a 

 sort of collar, or ring. From this ring springs a great number of 

 nervous cords, which are distributed to the mouth, the tentacles, the 

 lung, and the heart. The skin, in those parts covered by the shell, 

 exhibits great sensibility; it receives a considerable quantity of 

 nervous filament, so that the sense of touch ought to possess extreme 

 delicacy. 



The tentacles, the skin of which is so fine and so sensitive, are the 

 organs of touch. Other functions are sometimes attributed to them ; 

 the anterior tentacles are sometimes considered to be the organs of 

 smell. This, at all events, is certain, that the snail is very sensible 

 of strong odours, and is easily attracted by many plants the odour of 

 which pleases it. 



The black points which terminate the first pair of tentacles have 

 been considered as eyes : but the existence of a visual organ in the 

 snail is not quite certain. They are quite insensi ie to sudden 

 changes of light ; they always travel in the dark, and never recognize 

 obstacles placed before them. We may add that the pnail is destitute 

 of all organs of hearing. No noise appears to affect it, at least till 



2 D 2 



