. An introduction to zoology : for the use of high schools . s aggregated here, but there isdeveloped a complicated meclianism in connection with themouth, consisting of horny jaws and a lingual ribbon or radula,tlie surface of which is beset with teeth like a rasp or file, andwhich can be everted by special muscles. 20. Three-fifths of the Gastropods aie adapted for breathingair, the mantle-cavity being altered into a lung and the gills be-ing rudimentary (cf. VII, 12); they form an important order Pul-monata, a key to the structure of which is furnished by the pond-snail figui-ed above. It b
. An introduction to zoology : for the use of high schools . s aggregated here, but there isdeveloped a complicated meclianism in connection with themouth, consisting of horny jaws and a lingual ribbon or radula,tlie surface of which is beset with teeth like a rasp or file, andwhich can be everted by special muscles. 20. Three-fifths of the Gastropods aie adapted for breathingair, the mantle-cavity being altered into a lung and the gills be-ing rudimentary (cf. VII, 12); they form an important order Pul-monata, a key to the structure of which is furnished by the pond-snail figui-ed above. It belongs to a sub-order, the ofwhich (Basommatopliora) have the eyes at the bases of thetentacles, and possess thin shells, which may be spiral like Lim-ncea, or spiral and dexiotropous like PJujsa, or coiled in one plane like Planorhis, orsimply conical like An-cylus (Fig. 159). Morenumerous, however, arethe land-snails and slugswhich cai-ry the eyes atthe tips of the tentacles(^Stylominatoi^hora), andwhich, include shelledforms like HellXjZonites,. -Shells of fresh water Gasteropoda. 4 AnciiUin; b, ria,iis. Prosobranchs,-6, /o^M- ^.,(ccineaandfomiSWith,;mrt, with the operculum in the aperture; <,Gmuo. • _l i basiii. a rudimentary internal shell like Liniax. 21. A second order of Gastropods—ProsobrancMata—includes, for themost part, marine forms, differing from the rulmonata in possessing atrill in the mantle-cavity, and, usually, an operculum carried on the foot forclosing the aperture of the shell. The ordinal name is derived from thefact that the respiratory organ is situated in front of the heart, as it is inthe Puhnonata. Great variety of colour and form characterizes the shellsof ths Order, the Chitons, , having a shell formed of eight transversepieces, the Limpets (Patella), a simple conical shell, while endless va- HIGH SCHOOL ZOOLOGY. 235 rieties of spirals are to be met with in t]ie Top-shells, (Turbo and Tro-chus), Olives, Cone-shells,
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1889
