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Show 38 ON CIJASSIFICATION. diU'm, and metapodium ;*' while the Aplysim, in which the foot proper has the ordinary composition, exhibit pr?cesses from tho lateral and upper surfaces of that organ, hav1ng the form of great muscular lobes, which serve as a sort of aquatic wingR to some species, and arc termed epipodia. Tho Branchiogasteropoda are such of the Gasteropoda of Cuvier as breathe water either by means of the thin wall of the 111antle cavity (Atlanta, e.g.), or by special pallial branchioo (Pectinibranchiata, Tectibranchiata, &c.), or by certain parts of the integument of the body (Nudibranchiata) more or loss Fig. 15. Fig. 15.-Diagram exhiLiting the disposition of the intestine, nervous system, &c., in a common snail ( Ilelix). a, mouth ; b, tooth; c, odontophore; d, gullet; e, its dilatation into a sort of crop; f, stomach ; g, coiled termination of the visceral mass ; the letter is also clo;;e to the commencement of the intestine, which will be seen to lie under the oosophagns, and not over it as in the whelk; h, rectum; i, anus; 1~, renal sac; l, heart; m, lung, or modified pallial chamber; n, its external apertnrc; o, thick edge of the mantle united with thP. sides of the body; p, {oot; r, cerebral, pedal, and parieto-splanchnic ganglia aggregated round the gullet. specially modified. The next class, the PULMOGASTEROPODA, t on the other hand, are the Pulmonate Gasteropoda of Ouvier, * I proposed these terms in a Memoir in the "Morphology of tho Oophalous Mollusca," published in tho Philosophical Tmnsactions for 1853. t I adopt those convenient nnmes at the st1ggostion of my friend Professor Greene, of Queen's Oollogo, k. 'l'llE PULMOGA TEHOPODA AND P'l'EH PO A. 39 tho snail and sluQ: , whieh agree with the Branchiogasce'topocla in the o·en rn1 haractor~ of th -ir body, mantle, 11 rvmL' .. and respiratory systmn , and in po s ·in()' an (1ontophor ; but differ from th m, not only in br athing air by n1 an f tho thin lining of tho pallial ehmnb r, but, a I b lieve, by the dir ction of the iloxure of their int tin . A areful di - ection of a co1nn1on nail, for exa1nple (Fig. If), will pr v \ that, though the ann i, situated in the sa1n \Yay a in th Branchiogasteropocla, on the dor al r hannal ide of th body, the primary bend of th into tine i not to the hromal, but to the neural, side, the eventual tern1ination of tho inte tin on the hmmal side being tho result of a cond chango in it:s dir ·tion. }low far thi neural flexure of tho into tin' r ally pr vnil" among the Pulmo-ga te.ropocl is a qu tion which mu t L clecicl "cl by 1nore extensive investigation than I have a y t been enabl d to carry out. rrho members of the cla P'l'EUOPODA 0.1'0 Inall, or v ll 1niuutc, molluscs ; all rnarine in habit, and for tho most part p lagic, or swim1ners at the surface of deep s as. Like tho tw preceding groups, they pos e s three principal pair ~ of ganglia, an odontophore, a mantle, which i~ not di vi<l d into two loh , and whi ·h s cretes a univalve shell, if auy. But th propodin1n, mosopodium, and Inctapoclium are usually rudin1cutary, and locou1otion is almost wholly effect cl by tho epipodia, which are enonnously developed, and, in most of th g n ra, perfonu the office of aquatic wings still n1ore efficiently than thos of th Aplysim. Furthermore, tho intestine is flex d towards th n ural side of the body; and the head, with tho organs of ight, are usually quite rudimentary. I include in thi group n t only Oriseis, Oleoclora, Ilyalma, Pneumodermon, &c., but also tho aberrant genus Dentalium. * The last class of this s rie is that f tho OEriiALOrODAcomprising the Poulpes, the Cuttle-fishes, the Squids, and tho pearly N au til us ; a group definable by most marked ttnd distin ·t cbara ters from all the preceding, though it resemble thmu in * JJentulium resembles tho Pteropolla i.n its rudimcntnry 1Jt'at1, Uw nutmll1lexurc of itt:! iute tine, its cpipo lin.llohe~:~, null tl1 . tbanwtor of iiH larvn. |