STRUCTURE AND PiYSIOLOGY OF THE MOLLUSCA. pas 
branchs, whose stomachs are often remarkably branched, tho 
liver accompanies all the gastric ramifications, and even enters 
the respiratory papillz on the backs of the eolds. The exist- 
ence of a renal organ has been ascertained in most classes; in 
he b.valves it was dete. © by the presence of uric acid. The 
intestine is more dnyolutéd in the herbivorous than in the 
sarniyorous tribes: in the bivalves and in haliotis it passes 
through the ventricle of the heart; its termination is always 
near the respiratory aperture (or the excurrent orifice, where 
there are two*), and the excrements are carried away by the 
water which has already passed over the gills. 
Besides the orgens already mentioned, the encephalous 
molluscs are always furnished with well-developed salivary 
glands, and some haye a rudimentary pancreas ; many haye also 
special glands for the secretion of coloured fluids, such as the 
purple of the murex, the violet liquid of ianthina and aplysia, 
the yellow of the bullidw, the milky fluid of eolis and the inky 
secretion of the cuttle-fishes. The gland that secretes this 
# ad is situated on the mantle. It consists of a thin layer of 
“*ngated cells, and is to be found in most gasteropods. The 
uid produced appears to have different properties in different 
species. Thus in aplysia and some snails it possesses colour at 
the moment of being secreted; but in others it is colourless, as, 
for‘instance,-in turbo littoralis and trochus cinerarius. In murex 
and purpura also it is colourless when secreted; but on being 
exposed to the sun it becomes first yellowish and ultimately 
violet, after having passed through various intermediate tints 
formed by the mixture of yellow, blue, and red. According to 
~"'Lacaze Duthiers it is probable that the Romans obtained 
“> purple dye from three or four species of mollusc, such as 
wiuiee trunculus, and brandaris, and purpura heemastoma. A 
few molluscs exhale peculiar odours, like the garlic-snail (helix 
alliaria) an‘ eledone moschata. Many are phosphorescent, espe- 
cia “y the . vating tunicaries (salpa and pyrosoma), and bivalyes 
wh_-h inhabit holes (pholadide). Some of the cuttle-fishes are 
shghtly luminous; and one land-slug, the phosphoraa, takes its 
name from the same property. 
Circulating system. The mollusca have no distinct absorbent 
system, but the product of digestion (chyle) passes into the 
emeral abdominal cavity, and thence into the larger veins; 
* 1, Hst of the gasteropods the intestine returns upon icself, and terminates on the 
right side, near the head. Occasionally it ends in a perforation more or less removed 
from the)margin of the aperture, as in trochotoma, fissurella, macrochisma, and 
Gentaliui. In chiton the intestine is straight, and terminates posteriorly. 
