370 



COMPAEATIVE ANATOMY. 



Fig. 196. Vertical section through an 

 Anodonta. v Ventricle, a Auricles. 

 pp' Pericardial cavity, i Hind-gat. 

 m Mantle, hr hr' Branchia?. / Foot. 



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

 two limbs j which embrace the hind-gat (/), and which give off 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 auricle. 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 hamial 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 haemal spaces or 

 lacunas. 

 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 larger 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 periphery, 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 cither 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 haemal 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 



