STBUCTUEE AXD PlITSIOLOGT OF THE MOLLUSCA. 23 



branclis, wlioso stomachs are often remarkably branched, the 

 liver accompanies all the gastric ramifications, and even enters 

 the respii'atory papilloe on the backs of the eolids. The exist- 

 ence of a renal organ has been ascertained in most classes; in 

 the bivalyes it vas detected by the presence of ui"ic acid. The 

 intestine is more convoluted in the herbivorous than in the 

 carnivorous tribes : in the bivalves and in lialiotis it passes 

 through the ventricle of the heart ; its termination is always 

 near the respiratory aj^erture (or the excarrent orifice, -^here 

 there are t^^'o*), and the excrements are carried avray by the 

 water which has already passed over the gills. 



Besides the organs already mentioned, the encephalous 

 molluscs are always furnished with well-developed salivary 

 glands, and some have a rudimentary j9a?icre«s ; many have 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 hullidce, the milky fluid of eolis and the inky 

 secretion of the cuttle-fishes. The gland that secretes this 

 fluid is situated on the mantle. It consists of a thin layer of 

 elongated cells, and is to be found in most gasteropods. The 

 fluid produced appears to have difl'erent properties in diflerent 

 species. Thus in aplysia and some snails it j)ossesses colour at 

 the moment of being secreted; but in others it is colourless, as, 

 for instance, in tiirljo littoralis and froc//«s cinerarius. Inrnurcx 

 and 'puri-)ura 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 

 M. Lacaze Duthiers it is probable that the Eomans obtained 

 their pui'ple dye from three or four species of mollusc, such as 

 murex trunculus, and hrandaris, and purpura hcemastoma. A 

 few molluscs exhale jieculiar odours, like the garlic-snail {helix 

 alUaria) and eledone moschata. Many are phosphorescent, espe- 

 cially the floating tunicaries {scdpa and pyrosoma), and bivalves 

 which inhabit holes {pJwladidce). Some of the cuttle-fishes are 

 slightly luminous ; and one land-slug, the p)]iosp]iorax, takes its 

 name from the same j^roiDcrty. 



Circulating system. The mollusca have no distinct absorbent 

 sj'stem, but the product of digestion {chyle) passes into the 

 general abdominal cavity, and thence into the larger veins ; 



* In most of the gasteropods the intestine returns upon iiself, and teiTninates on tlio 

 right side, near tl\e head. OocapiongUy it ends in a perforation more or less rcmovcM 

 from the margin of the apcrtm-e, as in trochotoma, fissurella, macrochisma, and 

 dottaUwii. In chiton the intestine is .straight, and tenninatcs posteriorly. 



