The royal natural history . figure representing the male of one ofthe periwinkles (Littorina). Here therespiratory organs are lodged in a chamberformed by the mantle behind the head; thegills are variable in the different suborders,In some forms, with nonspiral shells, theyare double and symmetrical, but in others,and these are the majority, where the shellassumes a spiral form, the gill is usuallysingle. Some of the Prosobranchs havehorny jaws, but others are without; mostare furnished with a radula, but this issometimes absent. Characters derived fromthis organ have been largely employed int
The royal natural history . figure representing the male of one ofthe periwinkles (Littorina). Here therespiratory organs are lodged in a chamberformed by the mantle behind the head; thegills are variable in the different suborders,In some forms, with nonspiral shells, theyare double and symmetrical, but in others,and these are the majority, where the shellassumes a spiral form, the gill is usuallysingle. Some of the Prosobranchs havehorny jaws, but others are without; mostare furnished with a radula, but this issometimes absent. Characters derived fromthis organ have been largely employed inthe classifications of the suborders. Thesexes are separate; the head is distinct, andfurnished with a pair of tentacles, sometime*having the eyes at the ends, but mostly at ornear their base. An operculum, which isunknown among the Opisthobranchs amiPulmonates, except in the case of Avriphibola,is developed in the majority of Prosobranchs,but the number of genera in which thisIt assumes very different forms, and its size is. ANIMAL OF MALE PERIWINKLE, WITH THE SHELLREMOVED. a, Mouth ; b, Sexual organ ; c, Reflexed mantle ;d, Vent; e, Kidney ; /, Slime-gland ; g, Gill;h, Heart; in, Shell-muscle ; p, Foot. appendage is absent is considerable,very variable in proportion to the dimension of the aperture of the shell. In somevol. vi.—24 37o MOLLUSCS, genera it is as large as the aperture of the shell (Ampulla/ria, Natica, Bithynia),while in others it only partly closes it; and in other cases it is so small as to be ofno use for closing the shell. It is mostly horny, but in a few instances is more orless thickened and strengthened with a shelly or calcareous layer. The hornyopercula are mostly flattened, but in a few cases are thickened, or conical (Torinia,Coelopoma). The shells of Prosobranchs comprise some of the most beautiful, andmost prized by collectors. The order may be separated into three main divisions,namely, Pectinibranchiata, Heteropoda or Nucleobranchiata, and Scutib
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
