UNIVALVE MOLLUSC A. 421 



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 huinne, or air-cells, existing between the several organs. 

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



The snail has a rudimentary brain, composed' of a pair of thick 

 ganglions, situated above the oesophagus, which is in connection with 

 another pair of ganglions placed below, and these two together form 

 a sort of collar, or ring, around the oesophagus. From this ring 

 spring 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 number of nervous filaments, 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 

 odours of which please 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 still disputed by some. They are quite insensible to sudden 

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

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

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

 the noise is so near as to agitate the air which immediately surrounds 

 it. Indeed, the snail has few senses ; the poor creature is at once 

 blind, deaf, and dumb. 



The snails are male and female in the same individual, or herma- 

 phrodite. Their eggs are roundish, heavy, and of a whitish colour. 

 The animal deposits them on the soil in little irregular heaps ; at 

 other times it deposits them one after the other, like the grains of a 

 chaplet, in holes which it digs in the soil, or in the natural excava- 

 tions created by moisture. The eggs are even found in the hollows 

 of old trees, and in fissures of walls or rocks. 



When the young snail issues from the egg, it is already provided 

 with an extremely thin membranous shell. The timid and tender 



