30 MANUAL OF THE MOLLUSCA. 



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

 has ah'eady passed over the gills. 



Besides the organs already mentioned, the encephalous mol- 

 lusks are always furnished with well- developed salivary glands, 

 and some have a rudimentary j^a^zcreas ; many have also special 

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

 the rnurex, the violet liquid of iantlihia and aplysia, the yellow 

 of the hullad(S, the milky fluid of eolis, and the inky secretion of 

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

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

 phorescent, especially the floating tunicaries [salpa and pyrosoma), 

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

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

 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 

 are perforated with numerous round apertures. The circulating 

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

 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 

 body. From the capillary extremities of the arteries it collects 

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

 ratory organ, and returns to the heart as arterial blood. Besides 

 this systemic heart, the circulation is aided by two additional 

 branchial hearts in the cuttle-fishes ; and by foui- in the brachio- 

 poda. 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 the right side, near the head. Occasionally it ends iu a perfo- 

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

 toma, fissitreUa, macrochisma, and dentalium. Tn chiton the intestine is 

 straight, and terminates posteriorly. 



