370 



COMPARATIVE ANATOMY. 



Fig. 196. Vertical section throuo;li an 

 Anodonta. v Ventricle, a Auricles. 

 pp' Pericardial cavity, i Hind-gut. 

 w Mantle, hr hr' Branchiaj. / Foot. 



length. In most of the LameUibranchiata the heart is divided into 

 two limbs, whicli embrace the hind-gat {i), and which give ofJ the 



aorta after they unite. In Area this 

 passage of the hind-gut through the 

 heart leads to the formation of two 

 ventricles, which are represented 

 by two completely separate cham- 

 bers, each of which is provided with 

 an aui"icle. Each ventricle gives off 

 an aorta, which unites with its fellow 

 of the opposite side before it gives 

 off any branches, so that there is a 

 single aorta all the same. The same 

 holds for the posterior arterial trunk. 

 The anterior arterial trunk passes 

 as far as the region of the mouth, 

 where it gives off branches and opens 

 into wide hsemal spaces. The pos- 

 terior arterial trunk, the length of 

 which is dependent on the develop- 

 ment of the hinder portions of the 

 mantle, which represent the siphons, 

 also passes into htemal spaces or 

 lacunae. 

 Spaces of this kind, which are essentially marked off by connec- 

 tive tissue, ramify not only in the mantle, but also between the 

 viscera. They may be distinguished into lai'ger or smaller recep- 

 tacles for blood, according to their width ; while, further, they take 

 the place of a capillary and of a venous system. Larger sinuses 

 are regularly present at the base of the gills, and a median 

 azygos one — which collects together the venous spaces in the foot 

 — extends between the two adductor muscles. All of these haemal 

 spaces are connected together, and form in different parts a more or 

 less wide mesh- work. The two lateral spaces also communicate with 

 the organs of Bojanus (§ 290). 



If we follow out the course which is taken by the blood sent from 

 the arteries to the peripheiy, although, indeed, many points about it 

 have not been definitely made out, we find that some of it goes to 

 the mantle, and some to the visceral sac. Thence some of it passes 

 into the branchial sinuses, and from these either directly into the 

 gills, or to the glands of Bojanus, before it passes to the respiratory 

 organs. The latter course is that which is taken by most of the 

 blood. Since, however, the hsemal spaces at the base of the gills 

 are in direct communication with the auricles of the heart, part — 

 although a small part — of the blood is able to return to the heart 

 without going to the gills. The blood from the mantle also comes 

 to the heart, and passes at once into the auricles ; but this cannot be 

 regarded as absolutely venous blood, on account of the respiratory 

 function of the flaps of the mantle. As all the blood which comes 



