30 MANUAL OF THE MOLLUSC A. 





two*), and the excrements are carried away by tlie water wliicli 

 lias already passed over the gills. f 



Besides the organs already mentioned, the encephalous mol- ,' 

 lusks are always furnished with well-developed salivary glands, ; 

 and some have a rudimentary pancreas ; many have also special | 

 glands for the secretion of coloured fluids, such as the purple of 

 the murex, the violet liquid of iantJiina and aplysia, the yellow j 

 of i}iQ bulladce, the milky fluid of eolis, and the inky secretion of \ 

 the cuttle-fishes. A few exhale peculiar odom-s, like the garlic- | 

 snail {helix alliaria) and eledone moschata. Many are phos- ; 

 phorescent, especially the floating tunicaries {salpa and pyrosoma), j 

 and bivalves which inhabit holes ( pholadidcB) . Some of the cuttle- \ 

 fishes are slightly luminous ; and one land-slug, the pJwsphorax, \ 

 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 ge- \ 

 neral abdominal cavity, and thence into the larger veins, which j 

 are perforated with numerous round apertures. The circulating j 

 organs are the heart, arteries, and veins ; the blood is colourless, j 

 or pale bluish white. The heart consists of an auricle (sometimes \ 

 divided into two), which receives the blood from the gills ; and ? 

 a muscular ventricle which propels it into the arteries of the J 

 body. 1^'rom the capillary extremities of the arteries it collects ] 

 again into the veins, circulates a second time through the respi- 

 ratoi-y organ, and returns to the heart as arterial blood. Besides 1 

 this systemic heart, the circulation is aided by two additional i 

 branchial hearts in the cuttle-fishes ; -and^fgur in.th&4#J««4w>--. 

 .^Oifr. Mr. Alder has counted from 60 to 80 pulsations per 

 minute in the nudibranchs, and 120 per minute in a vitrina. 

 Both the arteries and veins form occasionally wide spaces, or 



* In most of the gasteropods the intestine returns upon itself, and ter- 

 minates on tlie right side, near the head. Occasionally it ends in a perfo- 

 ration more or less removed from the margin of the aperture, as in trocJio- 

 toma, psurella, macrochisma, and dentalium. In chiton the intestine is , 

 straight, and terminates posteriorly. 





