GASTEROPODA. 301 



position in the Doris and Testacella, and terminates on the left side 

 of the body in the Planorbis. 



The liver is a bulkj- gland, and is subdivided into numerous lobules 

 in all the Gasteropods : its secretion is derived from arterial blood, 

 and is usually poured by one or two ducts into the commencement 

 of the intestine. It is carried, however, into the stomach, and even 

 into the oesophagus, in the Onchidium : and the Haliotis not only 

 resembles the Palliobranchiata in the perforation of 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 auricle is divided in the Fissurella and in the Chiton as in the 

 Haliotis. Both auricles, however, equally receive the oxygenated 

 blood from the respiratory organ, as does the single auricle in all 

 the other Gasteropods. The ventricle 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, as has been already explained in the 

 Introductory Lecture. * The aorta, continued from the apex of 

 the ventricle, divides into two principal branches in most of the 

 Gasteropods. The auriculo-ventricular aperture is usually defended 

 by two semilunar folds. The aorta, at its commencement, is fre- 

 quently strengthened and enlarged 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 large 

 venae cavae of the Aplysia are perfoi-ated by minute apertures, com- 

 municating with the cavity of the abdomen ; and the exterior of the 

 veins is provided with decussating muscular fibres, which probably 

 regulate the diameter of their "peritoneal communications. Cuvier 

 supposes that this structure has relation to an absorption or passage 

 of fiuid from the abdomen into the veins. The vessels conveying 

 the venous blood to the branchiae are continued from these veins 

 without the interposition of any muscular ventricle. 



The general modifications of the respiratory organs are indicated 

 by the characters of the orders of the present class already defined. 

 In the terrestrial Gasteropods the breathing organ has the form of the 

 simple undivided vascular sac, like the lung in the lowest air- 

 breathing Vertebrate animals. The forms of the aquatic breathing 

 organ are as various as its position. 



In most of the Nudibranchiate species the gills are tufted and rami- 

 fied, as in the higher Anellides ; they are penniform in the Haliotis, 



* Page 6. 



