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SUPPLEMENT 
number of vessels, which proceed to unite in a ventral canal, 
situated underneath the intestine, and this canal distributes it 
to the entire body, from whence it returns to the heart, through 
a thick vena cava. 
Thus the circulation is double, as the heart must be con- 
sidered as the pulmonary ventricle, and the ventral canal as 
the aortic ventricle. 
In the squillae, the heart is elongated into a thick fibrous 
vessel, passing along the back and the upper part of the tail. 
That of the small entomostraca, such as daphnis, lynceus, 
limnadia, is small, globular, situated near the back, above the 
intestinal canal, and its contractions are very perceptible. 
In the limulse it is a thick vessel, furnished within with 
fleshy columns, running along the back, and giving out, like 
that of the squillae, branches from both its sides. 
Finally, in branchipes, we see from the head as far as the 
last articulation but one of the tail, a brilliant organ, perfectly 
diaphanous, which is composed of a series of utricles or little 
cells, corresponding in number to that of the rings of the body 
(eighteen or nineteen), which contract and dilate successively 
with considerable quickness, with movements which may be 
compared to those of the systole and diastole. This organ is 
very analogous to the dorsal vessel in insects. 
Respiration is a very active function in the Crustacea. The 
respiratory organs are voluminous, and of two kinds, gills or 
branchial laminae, and kinds of air sacs. 
The gills are sometimes concealed, sometimes visible; they 
are frequently situated on the sides of the body, but often also 
on the posterior extremity of its inferior face. They are 
almost always annexed to the base of the ambulatory feet, or 
to that of the most exterior parts of the mouth ; but also, in 
many cases, they of themselves alone constitute feet, which 
serve at the same time for locomotion and for respiration. 
In the brachyurous decapod Crustacea, they are placed at 
