30 MANUAL OP THE MOLLUSC A. 



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

 has already passed over the gills. 



Besides the organs already mentioned, the eiicephalous 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 ianthina and aplysia t the yellow 

 of the bulladce, the milky fluid of eolis, and the inky secretion of 

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

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

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

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

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

 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. Prom 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 four 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 in a perfo- 

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

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

 straight, and terminates posteriorly. 



