-MOLLUSCA. #1 
body. Any one may have ocular evidence of the 
existence of this organ, by watching our common 
Garden Snail. If you look at its right side, just 
behind the tentacle, or horn, that carries a black 
eye at its point, you will see a large hole suddenly 
open, where before there was no trace of it. After 
remaining open for a few moments, the margin will 
leisurely contract again, until it is perfectly closed, 
and as invisible as before. This is the breathing 
orifice ; and during the interval that you saw it open, 
the aerial contents of the chamber were expelled, 
and a copious draught of fresh air was inspired. 
The process is repeated with tolerable regularity 
about once every fifteen seconds. 
The blood in the MoLuusca is thin, transparent, 
and colourless; or at most presents only a pale 
bluish-white hue. It is, however, contained in a 
system of distinct vessels, through which it cir- 
culates, having for the source of its motion a well- 
developed, complex, pulsating heart. 
Besides the system of vessels which carry the 
blood, there is another system, most conspicuous in 
the aquatic tribes, which has been called the system 
of aqueducts. They communicate with the ele- 
ment in which the animal lives and moves, and are 
filled with it at will, as the galleries and canals of 
a sponge are filled with the liquid in which it is 
immersed. The chief use of these water-canals 
appears to be the distension and expansion of the 
foot, to render it better fitted for locomotion, yet so 
as not to interfere with the privilege, essential to 
most of these animals, of withdrawing the whole 
of the body within a shell. Some of the marine 
Mollusca, when in a state of activity, protrude a 
soft foot, far exceeding in dimensions the whole 
