Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 GASTROPODA. 39 one gill only remains (fig. 516). The respiration of air is confined to some Prosobra/nchiata and to the Pulmonafa. In this case al> 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,
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 GASTROPODA. 39 one gill only remains (fig. 516). The respiration of air is confined to some Prosobra/nchiata and to the Pulmonafa. In this case al> 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). N FIG. 516.—Anatomy of Cassis cornufa (after Quoy)). -ff, proboscis; Si, siphon; Sr, gill; jy/f, olfactory organ (formerly regarded as a rudimentary gill); Spd, salivary gland ; -ZV, 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 Opist/wbranc/iiata, 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 The kidney (fig. 516) is the most important excretory organ of
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