GASTROPODA. 39 



one gill only remains (fig. 516). The respiration of air is confined 

 to some Prosobranchiata and to the Pulmonata. In this case also 

 the mantle cavity serves as the respiratory cavity, but it differs from 

 the branchial cavity by containing air, and possessing, instead of a 

 gill, a rich network of blood-spaces and vessels on the inner surface 

 of its roof. Both branchial and pulmonary cavities communicate by 

 a long slit along the mantle edge or by a small round aperture, 

 capable of being closed, with the external medium. Frequently, 

 however, the edge of the mantle is prolonged into a long respiratory 

 tube of variable length, which is analogous to the siphon of the 

 Lamellibranchiata. This siphon corresponds, as a rule, to a notch or 

 canal of the shell (vide p. 32). 



rfTTTl/yiiijjuiii**. 



Br 



Nh 



N 



FIG. 616. Anatomy of Casnis cornuta (after Quoy)). It, proboscis; Si, siphon; Br, gill; 

 Nk, olfactory organ (formerly regarded as a rudimentary gill) ; Spd, salivary gland j N, 

 kidney ; P, penis. 



The structure of the respiratory organs has become of importance 

 for the classification of the larger groups. According to the position 

 of the respiratory organs, with regard to the heart and its auricle, 

 two great divisions can, as Milne Edwards has pointed out, be 

 established: (1) the Opisthobranckiata, in which the auricle and 

 gills are placed behind the ventricle; (2) the Prosobranchiata, in 

 which the auricle, with the branchial vein entering from the front, 

 lies in front of the ventricle. As far as this character is concerned, 

 the Heteropoda and most Pulmonata are allied to the latter group ; 

 but the Pulmonata, in many features of their organization and in 

 their hermaphroditism, stand closer to the Opisthobranchiata. 



The kidney (fig. 516) is the most important excretory organ of 



