24 MANTTAL OF THE MOLLTJSCA. 



whicli are perforated witli numerous round apertures. Tlie 

 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 respiratory 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. 

 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 sinuses ; in 

 the cuttle-fishes the oesophagus is partly or entirely surrounded 

 by a venous sinus ; and in the acephala the visceral cavity itself 

 forms part of the circulating system. 



Aquiferous system. Eecent anatomical researches by Messrs. 

 Hancock, Eolleston, Eobertson, Williams, and others have 

 thrown considerable doubt upon the existence of any aquiferous 

 system in the moUusca. There are certainly a number of pores 

 which open to the external water ; these are situated either in 

 the centre of the creeping disc, as in cyprcea, conus, and ancil- 

 laria ; or at its margin, as in haliotis, doris, and aplysia. In 

 the cuttle-fishes they are variously placed, on the sides of the 

 head, or at the bases of the arms ; some of them conduct to the 

 large sub-orbital pouches, into which the tentacles are retracted. 

 According to Messrs. Eolleston and Eobertson* there is no con- 

 nection between the blood vascular and the aquiferous systems; 

 and the foot in the lamellibranchiates is distended by means of the 

 aquiferous canals, which they regard as a rudimentary kidney. 

 Agassiz and Lacaze Duthiers, on the other hand, assert that there 

 is a connection between the two systems. The proof relied on 

 by the former observers was that when a coloured injection was 

 forced in through a vein, and an injection of a difierent colour 

 was sent into the aquiferous canals, two coloured systems of 

 ramification were formed, which the microscope showed to be 

 distinct up to the furthest extremities. Agassiz also used a 

 coloured injection ; he states that when it was injected through 

 the large pore in the pedal surface of some species of pyrula, 

 not only was the system of canals in the foot filled, but also the 

 whole of the circulatory system. He also states that when a 

 mactra is taken out of the water it discharges a quantity of 

 fluid from the foot, which consists of salt water, in which floats 

 * Philosophical Transactions, 1862. 



