24 MANUAL OF THE MOLLUSCA. 
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 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 yeins form occasionally wide spaces, or sinuses ; in 
the cuttle-fishes the cesophagus is partly or entirely surrounded 
by a venous sinus ; and in the acephala the visceral cayity itself 
forms part of the circulating system. 
Aquiferous system. Recent anatomical researches by Messrs. 
Hancock, Rolleston, Robertson, Williams, and others haye ~ 
thrown considerable doubt upon the existence of any aquiferous 
system in the mollusca. 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 cyprea, 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. Rolleston and Robertson* there is no con- 
nection between the blood vascular and the aquiferous systems; 
andthe foot inthe 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 different 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. 
