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Show 34 ON CLASSII•'ICATION. 1nussel (Anodon ), is included within a mantle or" pallitnn," which is formed by a prolongation of the dorsal integument,-a structure in principle quite simila~ to .that w~ich ~ve Inet with in tho Brachiopoda. But there IS th1s essent1al d1fference between the two,-that whoroas, in the Bracltiopoda, the mantle lobes corresponded ·with the anterior and posterior re?;ions of the body, in the Lamellibranchiata they answer to the nght and left hal vos of the body. The intestine, which always terminates by a definite anus between the mantle lobes, at the posterior end of the body, has its first flexure neural. There is always a well-developed heart, which is 1nuch n1ore complex than that of the Ascidians or Brachiopods, being divided into distinct auricular and ventricular chambers. Commonly, there are two auric.I s [ll1d ono ventricle, as in Anodon; but in other Lamellibranchiata, such as the oyster, there is a single auricle and a single ventricle, and in some exceptional cases there are two auricles and two ventricles, forming two distinct hearts. Distinct respiratory organs, wl1ich usually have the forn1 of lamelJm (as the nnmo of the class Fig. 13. A '.\ \ \ \ \ \ \ \1 ! I I; /; (;I A Fig. 13.-Anodon, Yertical and transverse section of the body tl1rough the heart; f, ventricle; g, auricles; c, rectum; p, pericardium; h, inner, z', outer gill; o'q, orgau of Bojanus ; B, foot; A .A, mantle Jobef:. implies), are found in all Lamellibranchiata, and are situated upon each side of the body, in a chamber which oxtonrls between THE BTIANCIIIOOASTEROPODA. 3 the foot and tho manti lobo in front, and botwo n the n1antl lobo post riorly (Fio-. 13). The branchial organ may con .. i t f di tinct filaments, or of plates compos d of tubular rod upporting a network of blood-vessels: and cov r I with ilia, by th action of which they are constantly bathed by current of wat r. Tho nervous syst 'm pr ·nt a no le. s eli tinct ad van than the other organs. All Lmuollibran hs po ess at 1 a t thr pairs of principal gano·lia-a corrbral pair at th sid f th mouth, a })edal pair in the foot, and a third pair on tho un l r surface of tho posterior addn tor mu cl , which ar omm nly called" bran hial," bnt which, a th ey supply not only bran hia1, but vi .. ceral and pnJlial fHarnonts, may more properly bo t rm d "parieto-splanchnic." 'J.1hreo sets of com1nis ural filament onnect tho cerebral ganglia with one another, with tho pedal, and with the parieto-splanchnic ganglia. Th inter-cerebral connnisSlues surround the mouth; an(l tho other two pair· of cord · extend respectively, from tho cerebral to the p dal, and frOJn the c rebral to the parieto-splanchnic ganglia. Finally, there is alway , in th e animal , an xt rnal . holJ, which .is formed as an xcr tion from the surface of the lobes of the mantle, and is compo ocl of layer of animal matter l)ardm1ed by deposit of carbonate of li1ne, which may or may not tal o a definite form, and so give 1·ise to "pri 1nati ·" and "nacreou ·" substance. As tho lobes are right and l ft, so the valv s of the sh ll are rio·ht and left, and differ altogoth r fro1n tho valves of the shell of tho Brachiopoda, which are anterior and po tori01·. The val res of the hell can be brought tog th r Ly addu tor 1nusclos. Of these ono (Fig. 12, II) always exists, posteriorly, on the ueural side of the intestine. A second (Fig. 12, I) iH cmn1nonly found anteriorly to the mouth, on the hromal . ide of tl1e intestine. ~rhe hiatus between the next class, which is termed BnA - CIIIOGASTEROPODA in the table~ and that just d fin d, i ~ con icierable, though 11ot quite o . well mark d a that b tw n tho A cidian and the Lamellibranchiata. Thi .. group, which contains the whelks, periwinkles, sea-slugs, and th Ileteropoda f Cuvior, con ists of animals which, like the Lam llibran ·h , po - sesR (in th ir young tate, at any rate) a Incmtl ; a foot, 1vl 1 ieh J) 2 |