PTEROPODA AND GASTROPODA. 559 



the stomach by the bile-ducts, but likewise in the perforation of the 

 ventricle of the heart by the rectum, and in the division of the 

 auricle into two cavities. 



The blood of the Gastropods is often opalescent, with a few colour- 

 less corpuscles or cells, having an indistinct granular nucleus. 



The auricle is divided in Fissurella and Chiton as in Haliotis. 

 Both auricles, however, equally receive the oxygenated blood from 

 the respiratory organ, as does the single auricle {fig' 207., o) in all 

 the other Gastropods. The ventricle (ib. p) propels the blood to 

 the viscera and muscular system of the body, and the heart is thus 

 systemic, co-ordinately with the condition of the muscular system 

 and the general endowments of the animal.* The heart is situated on 

 the right side of the back in the Pulmonata, most Tectibranchiata, 

 and the dextral Pectinibranchiata : it is on the opposite side in 

 Ancylus, Haliotis, and the sinistral Gastropods : it is to the left of the 

 dorsal median line in Carinaria ; and near the hinder end of the 

 body in Firola and Atlanta, The heart has a distinct pericardium 

 in all Gastropods save the Aptieiista, where it is not clearly defined. 

 The aorta, continued from tbe apex of the ventricle, divides into 

 two principal branches in most of the Gastropods. The auriculo- 

 ventricular aperture is usually defended by two semilunar folds. 

 The aorta, at its commencement, is frequently strengthened and en- 

 larged by a muscular layer, similar to the bulbus arteriosus in fishes, 

 and which, in the Aplysia, is continued beyond the origins of the 

 primary branches of the aorta. The ramifications of the aorta, as in 

 Crustaceans and Insects, are sooner or later lost in veins which 

 expand to form sinuses, occupying the lacunae of the viscera and other 

 organs of the body.f The anterior aorta terminates, e. g.y in Patella, 

 Triton, Haliotis, in a large lacunar sinus containing the brain, the 

 salivary glands, the oesophagus and retracted tongue. The resump- 

 tion of the normal vascular character by the venous system is more 

 or less sudden : and is best exemplified near the respiratory organ 

 upon which such venous trunk ramifies, like an artery, without any 

 interposed branchial or pulmonic heart. The large vence cavoe of the 

 Aplysia, e.g., are perforated by minute apertures, communicating 

 with the great sinus that lines the cavity of the abdomen ; and the 

 exterior of those veins is provided with decussating muscular fibres, 

 which probably regulate the diameter of such communications. The 

 diff'used condition of the vascular system most prevails in those Gas- 

 tropods in which the respiratory organs are least developed, e.g., 

 the Apneusta. In the rest of the class the general modifications of 



* LXXXIV., p, 7. t CCCXLL, CCCXLVII. 



