UNIVALVE MOLLUSCA. 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 /acun@, 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 cesophagus, which is in connection with 
another pair of ganglions placed below, and these two together form 
a sort of collar, or ring, around the cesophagus. 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 isso 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 
