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POPULAR SCIENCE REVIEW. 
carried, moulded into threads by a viscid secretion, along a 
kind of groove formed by the free edges of the latter organs, to 
the mouth, into which they are guided by the tentacles. 
Decomposed animal and vegetable matter forms the proper 
food of these molluscs ; and “ in ponds,” says Dr. Gray, “ where 
there is plenty of food (and a dead dog, or cat, or fish affords 
abimdance of such materials), and where the water is nearly 
stagnant and seldom disturbed, they become of a large size.” 
Circulatoi'y System . — The anatomy and physiology of the 
organs of circulation is somewhat complicated, and has occu- 
pied the attention of numerous observers. The centre of the 
circulation, the heart (fig. 4, h ) lies at about the middle of the 
dorsal region of the body, immediately beneath the junction of 
the mantle-lobes. It consists of a single muscular ventricle 
(fig. 6, v\ through which runs the terminal portion of the 
intestine (see figs. 2 and 3, and fig. 6, i) ; and on the floor of 
which are two orifices, each of which communicates on either 
side with a triangular membranous auricle (fig. 6, a u), and is, 
moreover, guarded by an eyelid-like valve to prevent reflux of 
blood. A large pericardial sac (figs. 4 and 6, p) surrounds the 
whole. From the anterior end of the ventricle is given off an 
aorta, which at about the point of emergence of the intestine 
from the foot mass takes a direction inclining to the right, and 
keeps for some distance not far from the dorsal surface of the 
body. 
Immediately beneath the pericardium is situated an organ, 
called, after its celebrated discoverer, the “ organ of Bojanus,” 
(figs. 1, 2, and 3, r). It is made up of two symmetrical lateral 
factors, between which lies a venous sinus (fig. 5, v s), each 
further consisting of two sacs which lie one above the other. 
The upper, or “ pleural sac” (figs. 4, 5, 6, p s), roofed in by 
the floor of the pericardium, and communicating with its fellow 
for the anterior third of its extent, has smooth walls, while the 
walls of the lower, or “glandular sac” (figs. 4 and 6, gs), which 
is of a dark brown colour, are thrown into numerous large 
glandular folds. Anteriorly, each pleural sac communicates 
with the branchial chamber by an orifice (figs. 4 and 5, x), 
while posteriorly it is brought into relation with the glandular 
sac by a large fissure (figs. 4 and 5, y ), which is somewhat 
masked by certain of the folds just described. The glandular 
sac, moreover, of either side communicates anteriorly with the 
pericardial space by an orifice (fig. 4, z), guarded by a valve 
which only allows of the easy passage of fluid from the former 
into the latter cavity. 
The circulation in the river-mussel and its allies is, accord- 
ing to Milne-Ed wards, lacunar ; that is, the blood-vessels, as 
they recede from the heart, gradually lose their walls, and 
