206 COMPARATIVE ANATOMY chap. 



sinus, whicli lies at the base of tlie circle of gills. From this the blood rises into the 

 gills, becomes arterial, flows back into an outer circumanal vessel, and thence back 

 through the branchial vein into the auricle (Fig. 93, p. 98). 



Nudibranchia. — The heart, enclosed in the pericardium, almost always Ues in 

 front of the centre of the body, in the median line. The aorta, which springs from 

 the ventricle, divides into an anterior and a posterior aorta, each of which breaks up 

 into an arterial system, the arteries having walls of their own. The finer branches of 

 these arteries open into the lacunar system of the body, which occasionally fonns 

 canals resembling vessels, and is connected with the large cephalic and visceral 

 sinuses. Veins, apparently with walls of their own, run from the lacunar system of 

 the dorsal appendages or the integument, and carry the arterial blood back to the 

 auricle. The blood usually finally enters the heart through three "branchial" veins, 

 — two lateral and one median posterior, — Avhich open into the posteriorly-placed 

 auricle. 



3. Scaphopoda. 



The circulatory system of Dentaliuin, but for the recently-discovered rudimentary 

 heart, is entu'cly lacunar, consisting of systems of canals, sinuses, and spaces, the 

 special arrangement of which cannot here be described. 



The pericardium with the heart lies on the posterior side of the body, dorsally to 

 the anus. If we imagine the intestine of Dentaliuin straight and horizontal, the 

 heart would occupy the typical position on the dorsal side of the hind-gut. It has 

 no auricles, and is merely a sac-like bulging into the pericardial cavity of its 

 anterior wall. It is connected by fine slits with the suiTounding sinuses of the body. 



4. Lamellibranehia. 



The Heart. — In nearly all bivalves, the heart, which is traversed by 

 the hind-gut, possesses tvi^o lateral auricles, and lies in a pericardium. 



There are, however, isolated exceptions to this rule. In Nucula, 

 Area, and Anomia, the ventricle lies over (dorsally to) the hind-gut. 

 This dorsal position must be regarded as the primitive position of the 

 Lamellibranchiate heart, since the above genera are among the most 

 primitive bivalves, and, further, since the heart of the Amphineura, the 

 Scaphopoda, and the Cephalopoda also lies over or behind the hind-gut. 

 The perforation of the heart by the hind-gut must have arisen by the 

 bending of the ventricle down round the latter. 



The heart in the above-mentioned genera is further distinguished by the fact 

 that the ventricle is more or less elongated in the transverse direction, its lateral 

 ends being swollen, while the central part, which Ues above the intestine, becomes 

 narrower and thinner. This modification goes furthest in Area Nom, where there 

 seem to be two lateral ventricles unconnected by a central portion. This separation 

 of the ventricle into two lateral parts has here brought about a separation of the two 

 aorta. The two anterior as well as the two posterior branches, however, after a 

 comparatively short separate course, unite to form an unpaired anterior and an un- 

 paired posterior aorta. 



Although these genera have, as a rule, a heart lying above the hind-gut, in some 

 specialised forms the heart is placed under the hind-gut, e.g. Meleagrina, Ostrea, 

 Teredo. The cause of this modification must lie in the increasing distance between 

 the base of the gills and the original region of the heart, the auricles and the ventricle 

 having shifted with the latter. The auricles, however, no longer lie laterally to the 



