Chap, v.] HEART OF MOLLUSCA. 191 



cardiac chambers is reduced, and in the mites appear 

 to be altogether absent. 



The circulatory system of the Mollusca presents 

 a remarkable difference from that of the Arthropoda, 

 in so far as the blood never passes into the peri- 

 cardiac sinus. The heart is again formed from part 

 of the dorsal vessel, and in the least modified forms, 

 or such as still present a bilateral symmetry, a pair, 

 or two pairs (Nautilus), of transverse vessels open 

 directly into the central or axial portion of the heart ; 

 the ends of these vessels nearest to the axial portion 

 are enlarged in size and modified to form auricles, 

 while the altered part of the dorsal trunk serves as a 

 ventricle, from which the blood passes forwards by 

 an anterior, and backwards by a posterior, aorta. 



A simple arrangement of this kind is well seen in 

 the mussel (Aiiodon), or in the squid (Loligo). What 

 is probably a still more primitive arrangement is 

 presented by the Nautilus, in which there are two 

 pairs of transverse vessels, and therefore two pairs of 

 auricles. In the Octopus the aortic vessel, which in 

 the mussel was directed backwards, now takes a 

 forward course, or at first runs parallel to the true 

 anterior aorta ; in those Gastropods that have suffered 

 a more or less well-marked torsion of the chief viscera 

 (see page 81), there is but a single auricle, and the great 

 vessel arising from the front end of the ventricle early 

 divides into two branches ; of these one, like the 

 anterior aorta of the mussel, supplies the front end of 

 the body, while the other is, in like manner, distributed 

 to the chief viscera. 



The circulation is, to a large extent, effected 

 in a manner which has been called lacunar ; but, 

 as has been already pointed out, our knowledge of 

 these lacuiife is in a very elementary condition. The 

 statement that water is taken up into the blood- 

 vascular system by pores in the foot does not appear 



