202 COMPARATIVE ANATOMY chap. 



This serial order of the ventricle, auricle, branchial or pulmonary vein and 

 respiratory organs is characteristic of the Azygohranchia, Monotocardia , and most 

 ridmonata. 



The DocogloKsa {Patella and allied forms) have only one auricle ; the ventricle 

 in Patella (not in Acmcea), however, is divided into two parts. 



Among the Monotocardia, only Cypra;a (as far as is at present known) has a 

 rudimentary right auricle, closed on all sides except at its aperture into the 

 ventricle. 



Among the Pulmonata there are forms in which the auricle lies behind the 

 ventricle. This nmst be regarded as a secondarily acquired position, determined by 

 the shifting back of the anus and the mantle cavity to the posterior end of the body 

 ( TestaceUa, Oncidium). In Daudehardia, the auricle still lies in front of the ventricle; 

 nevertheless this genus, like several other shell-less Pidmoitates, is opisthojmeumonic, 

 i.e. its respiratory network lies chiefly behind the heart. In TestaceUa, the auricle 

 also lies behind the heart {ef. p. 77). 



In the Opisthobranchia, the auricle lies behind the ventricle ; this is connected 

 with the iiosition of the gill at the posterior end of the body, or where no true 

 ctenidium is found, but where respiration takes place by means of anal gills, or 

 dorsal appendages, or through the integument, with the point of entrance of the 

 branchial vein into the heart from behind. 



In a few Tcctibranchia, e.g. Actaon, Acera, Gastropteron, the gill lies some- 

 what far forward, and tlie auricle is then placed laterally, to the right of the 

 ventricle rather tiian behind it. 



It is of great importance, with regard to the position of these 

 organs in the Lamellibranchia, to note the fact that, in many Diotocardia 

 {e.g. Fissurella, Haliotes, Turbinidm, Trochidce, Neritidoi, Neritopsidm, etc.) 

 the ventricle is traversed by the hind-gut, while in all other Gastropods 

 the intestine merely runs past it. 



Circulation. («) Prosobranchia. — A large vessel, the aorta, springs from the 

 ventricle. This soon divides into two branches : (1) tlie anterior or cephalic aorta 

 (A. cephalica), and (2) the posterior aorta (A. visceralis). 



The anterior aorta conveys blood to the anterior part of the body (head, pharynx, 

 proboscis, cesophagus, stomach, copulatory organs) and to the mantle, and gives off 

 among otliers the important arteria pedalis ; this latter soon breaks up into 

 separate arteries, which run longitudinally through the foot. In some cases the 

 cephalic aorta is richly branched, breaking iip into numerous fine vessels which 

 spread out in and on the above-mentioned organs ; in others, the arteries, without 

 branching, open into arterial sinuses. Among these, the large cephalic sinus into 

 which the anterior aorta opens {e.g. in Haliotis) deserves special mention. Whei-e 

 the cephalic aorta runs beyond the oesophageal ring formed by the central ganglia 

 and their commissures, it passes through this ring. 



The aorta visceralis supplies the organs which lie in the visceral dome, especially 

 the digestive gland, the genital glands, and the mid-gut. 



The venous blood collects in the lacunar spaces of all parts of the body, and 

 flows into a large venous sinus, i.e. into the space in which the stomach, salivary 

 glands, intestine, digestive gland, and genital organs lie. This space or primary 

 body cavity is somewhat spacious round tlie stomach, but very limited in the 

 visceral dome, where the lobes of the digestive gland, the walls of the intestine, 

 and the genital glands with their accessory parts are so crowded together as to leave 

 very narrow spaces between them. 



