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CIRCULATION IN MOLLUSOA. 251 
takes place nearly on the same general plan as in Fishes ; the 
heart having two cavities, and the whole of the blood travers- 
ing both the respiratory and the systemic vessels, between 
each time of its leaving the heart and returning to it again. 
But this heart is systemic, and not pulmonary ; for it receives 
the arterial blood from the gills, and transmits it to the great 
systemic artery ; and after the blood has been rendered venous 
by its passage through the tissues of the body, it enters the 
channels which distribute it to the gills, before being again 
subjected to the action of the heart. The accompanying figure 
(fig. 139) of the circulation in the Doris (a kind of sea slug) 
Fig. 139.—CrrncuLatise APPaRatus oF Doris. 
will serve to show the general distribution of the vessels in 
this group. The heart consists of the ventricle a, whence 
issues the main artery 6; and of a single or double auricle ¢, 
_ in which terminate the veins, d, of the branchial apparatus e. 
_ The aerated blood which these convey to the heart, is trans- 
mitted by it, through the artery 6, to the system at large ; and. 
from this it is collected, in the state of venous blood, by the 
‘sinuses which terminate in the large trunk ff, By this trunk it 
