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Show 36 ON CLASSIFICA'fiON. is the chief organ of locomotion ; and three principal pairs of ganglia-cerebral, pedal, and parieto-splanchnic. When they are provided with a heart, which is usually the case, it is divided into auricular and ventricular chambers; but the 1nantle, instead of being divided into two htterallobes, is continuous round tho body, and wl1en it secretes a shell from its surface, that shell is either in a single piece, or the pieces are repeated from before backwards, and not on each side of the median line. The sholl of a Branchiogasteropod n1ay, therefore, be univalve, or composed of a single conical piece, straight or coiled; or it may be 1nultival ve-formed of a nun1ber of segments succeeding ono another antero-posteriorly; but it is never bivalve. Someti1nes a shelly, horny, or fibrous secretion takes place fron1 the foot, giving rise to a structure resen1bling the byssuR of some Lamellibranchs; it is the so-called "operculun~, " which serves to protect the animal when retracted into itH shell ; but as the operculun1 is developed from the foot and not fl·om the mantle, it can obviously have no homology with the valves of either a Brachiopod or a Lamellibranch. The Branchiogasteropoda (}j-.,ig. 14) com1nonly possess a distinct head, provided with a pair of tentacles and a single pair of eyes, both of which are supplied with nerves from the cerebral ganglia. Cephalic eyes of this kind are not known in the L amellibranchiata. But the characters which most definitely distinguish the B ranchiogasteropoda are to be found in the alimentary canal. The cavity of the n1outh is invariably provided with an organ which is usually, though not very properly, called the tongue, and which might more appropriately be denominated the "odontophore." It consists essentially of a cartilaginous cushion, supporting, as on a pulley, an elastic strap, which bears a long series of transversely disposed teeth. The ends of the strap are connected with 1nnscles attached to the upper and lower surface of the hinder extremities of the cartilaginous cushions ; and these muscles, by their alternate contractions, cause the toothed strap to work, backwards and forwards, over the end of the pulley formed by its anterior end. The strap consequently acts, after the fashion of a chain-saw, upon any substance to which it is applied, and the resulting wear and tear of its anterior teeth THE BRAN IIIOGA, TEROP DA. 37 are made good by the in e ant devel pm nt of new t th in the e01·eting ac in which tho hinder ncl of the trap i loclo· cl. Be.,icle th . chain- aw-like 1notion of the trap, th oclontoph ro n1ay be capable of a licking or scraping action a a whole. Fig. 1 4.-~ecti ou of a female whelk ( B~tccinwn) . The organs marked t and It art> remove I from their proper places ; t he oth ers arP. seen in ittt. a, month; b, gullet ; c, head ; d, foot; e, opercu lum; f, free part of t he mant le ; ,g, t hat part which in vests the vise ral mass lodged within the shell; h, a g land of unkn own function connected with the gull et ; i, crop ; ll, stomach ; l, intes tine; 1n, r ectum ; n, heart; o, ape rt ure of th renal organ ; r, mucous gland developed f rom the wall of the mantle cavity ; s, oviJuct; t, sali vary gland. The anows indic.:'lte the posit ion of the_ branchire. The cerebral, pedal, and pariet?-splanchnic ganglia closely surround the . g ullet, and t he latter send off a long ganglwnateJ cord towarJs t he hear t and branclure. The other peculiarity of the alimentary canal of th Branchiogasteropocla is that it is always bent upon itse1f, at :first, not to the neural, but to the hremal, or heart side of the body~ the rectum very commonly opening into the mantle cavity, above the cephalic portion of the Locly. In most B ranchiogasteropoda the foot is a broad, flat, mu ·cular body, without any distinct division of parts ; but in so1u ' fonns, uch as the Heteropocla of Ouvier, it is divicl d into tln c very well-marked portions-an anterior, a Iuidcll and a p ·terior, which ar t rmecl respectively the propodium, mesopo- ~ - |