They consist of a row of flattened filaments bound together at the base like the leaves of a book. The blood is conveyed to these gills by a large vein, and is then forced through the thinly walled filaments, being thus brought into close contact with the water, to which it releases its carbon dioxide, and from which it receives the life-giving oxygen. In certain gasteropods there are two sets of gills, one placed upon each side of the body [pg338] (as in the ideal mollusk); but the presence of two sets of gills only indicates, in these gasteropods, that the twisting process of the viscera, already alluded to as so remarkable a feature of this class, has not proceeded far enough to destroy the functional activity of one of the gills. [Illustration: Anatomy of the internal organs of a female gasteropod, viewed from the dorsal side. The roof of the mantle cavity has been divided by a longitudinal incision and the flaps laid out, that on the left bearing the ctenidium and osphradium, and that on the right the rectum and terminal part of the oviduct. The muscular dorsal wall of the body and the introvert have been divided so as to bring into view the anterior part of the alimentary canal and a portion of the nervous system. The buccal cavity has been tilted up and opened so as to show the odontophore, and the esophagus has been cut through near the anterior end. A portion of the ventral wall of the crop has been removed so as to bring the internal folds into view, and the interior of the nephridium with the contained portion of the intestine has been exposed. The stomach is not seen, being hidden by the nephridium, and the ovary is not represented. /an./, anus; /ant. aort./, anterior aorta; /aur./, auricle; /buc./, buccal cavity; /cer. buc. con./, cerebro-buccal connective; /cer. g./, cerebral ganglia; /crop/, crop; /cten./, ctenidium; /int./, intestine; /jaw/, jaw; /l. buc. g./, left buccal ganglion; /l. sal. gl./, left salivary gland; /neph./, nephridium; /neph. ap./, nephridial aperture; /od./, odontophore; /oes./, esophagus; /oes'./, anterior end of same, cut and turned aside; /osph./, osphradium; /ovid./, oviduct; /ovid'./, terminal thick-walled portion of oviduct; /pleur. g./, pleural ganglion; /post. aort./, posterior aorta; /post. oes./, posterior esophagus; /rad. s./, radula sac; /r. sal. gl./, right salivary gland; /rect./, rectum; /sal. du./, salivary duct; /siph./, siphon; /supra. g./, supra-esophageal visceral ganglion; /tent./, tentacle; /vent./, ventricle.]