. A manual of zoology. Zoology. IV. GASTEROPODA; PROSOBRANCHIA 333 In the development the great constancy with which the veliger stage (figs. 313, 314) appears is noticeable. Most marine Iarv£c swim at the surface by their velum before creeping at the bottom. But in those cases where the snail leaves the egg in the adult form the velum is usually developed in embryonic life, sometimes so strongly that the embryo rotates in the surrounding ihiid. « Order I. Prosobranchia. In the prosobranchs the twisting of the body has brought the ctenidia and consequently the auricles in front of the ventricl
. A manual of zoology. Zoology. IV. GASTEROPODA; PROSOBRANCHIA 333 In the development the great constancy with which the veliger stage (figs. 313, 314) appears is noticeable. Most marine Iarv£c swim at the surface by their velum before creeping at the bottom. But in those cases where the snail leaves the egg in the adult form the velum is usually developed in embryonic life, sometimes so strongly that the embryo rotates in the surrounding ihiid. « Order I. Prosobranchia. In the prosobranchs the twisting of the body has brought the ctenidia and consequently the auricles in front of the ventricle, and hence the aorta leads backwards. It also results in the twisting of the nervous system like the figure 8. The sexes are separate and the shell and mantle are usually well developed. Certain Prosobranchs are near the primitive Amphineura in the retention of both ctenidia, both auricles, and both nephridia, but in the great majority only one gill (the primitive right) is present and the corresponding auricle alone is well developed, although the other may exist in a rudimentary condition. z. Fig. 340.—Fissurella palai^onira, Fig. 341.—Actiiwa Ic^liidinalis*'lira-pet ventral view (from Bronn). br, the (from Ijinncj'-Gould). paired gills; p, foot. Sub Order I. ASPIDOBRANCHIA (Diotocardia), Ctenidia bipectinate (fig. 340) or absent. Usually two auricles and two nephridia. DOCOGLOSSA (limpets), auricle single; intestine not passing through heart, shell conical. AcM^ with ctenidium. Acnicca* (fig. 341). Patellid.!, ctenidia replaced by a ring-like mantle gill. ZYGOBRx\NCHI.\. Two ctenidia (fig. 340), shell with marginal slit or with holes; auricles and usually nephridia paired; heart traversed by intestine. ;, keyhole limpets; shell conical, with apical opening. , abalones (fig. 330, B). AZYGOBR.\NCHIA. One ctenidium, two auricles. , Trodms, Margarita* , top shells; Sub Order II. PECTINIBRANCHIA (Monotocardia). Ctenidium uni-
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
