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Show 26 ON CLASSIFICATION. but is bent upon itself (Fig. 8), and the direction of flexure is such that the nervous gang1ion, which corresponds with those called "pedal" in Lamellibranchiata, is placed in the re-entering angle between the gullet and the rectum. In order to express Fig. 8. Fig. 8.-Plumatella repens, a single cell more magnified; m, calyx at the base of the ciliated tentacula borne by the disk or 1ophophore; ll, gullet; g g, stomnch; It, intestine; i, anus; w, nervous ganglion (after Allman). this relation of the nervous system to the alimentary canal, the flexure of the latter has been called ":p_eural "-the side of the body on which the principal ganglion is placed, and towards which the intestine is bent, being the "neural " side. Whatever our terminology, however, the great point is to rmnember that the structural relation which it expresses is constant throughout the P olyzoa. In the next division, the BRACHIOPODA, which are animals differing very much in external appearance from the Polyzoa, 'riiE BRACHIOPODA. 27 we shall find, neverthele s, a singular fnndamental r semblanc of internal tructure to the latter. All known Polyzoa are compolmd animals, that is to say, the product of every ovum o·ive rise, by gemmation, to groat a se1nblag s of partially ind prudent organisms, or zooids. The Brachiopoda, on the contrary. are all simple, the product of each ovum not giving ri. to ot h rR by gemmation. All tho Brachiopoda po. soss a bivalve sh 11-a shell composed of two, more or less horny, or calci:fi d, pier , which are capable of a certain range of motion on one anoth r, and are very commonly articulated together by teeth and sockets. The proper body, which is small when compared with the size of the shell, has its dorF:al integument produeed into broad membranous expansions, which line the interior of the valves of the shell, and are callerl the lobes of the mantle, or "pallium." The aperture of the mouth is situated in the middle line, Letv\'oen the pallial lobes, and, on each side of it, is a longer or shorter prolongation of the body, provided with ciliated tentacula. It is fr01n the presence of these "arms" that the class has received its name. The tentaculate oral disk of a Plurnatella is already horse-shoe shaped (Figs. 7 and B); suppose each crus of the horse-shoe to be pulled out to a much greater length, and tontaculated "arms" would be produced, closely resen1bling those of the Brachiopoda. The mouth leads into a gullet which is directed towards, or lies along, that side of the body from which one lobe of the mantle, the anterior, is continued; the gullet opens into a stomach, provided with a well-developed liver; and from the stomach, an intestine proceeds, which is directed towards, or along, that side of the body from which the other lobe of the mantle proceeds; and then either, as I pointed out some years ago,* ends, blindly, in tho middle line (Fig. H), or else terminates in a distinct anus between the pallial lobes. * Prof. ssor Owen, in the s con<l edition of his lectures on il1e "Comparative Anatomy and Physiology of tl1c Invert bmtc Animals," puhlisbcd in 1855, thought it not unbecoming to sneer at this eli covcry. ''There mny be blindness somcwlwr , but I think not at the termination of tho intestine of Te1'ebmtula."-h c., p. 403. As my Htatcmcuts have subsequently been fully borne out by Mr. Albany Hancock and by M. J_,acaze Duthiers- two of the best minute anatomists of tho clay-l trust Mr. Owen is now fully Ratisfiecl as to where the "blindncAs" rcnlly was, in 18fi!>. |