| OCR Text |
Show ~8 ON CLASSIFICATION. The principal ganglionic mass is situated behind and below the mouth, in the re-entering anglo between the gullet and the rer.tum; in other words, the intestine has, as in tho Polyzoa, a neural flexure (Fig. 9). In all Brachiopoda which have Lecn carefully disseqted, a singular system of cavities and canals situ· ated in the interior of the body, but in free communication with the surrounding n1edium, has Lcen discovered. ThiR, which I 'l'IIE IH1ACHIOPODA. 20 hall tenn the "atrial" syst m, from its close correspondenc with tho systmn of envities, which ha. received tho smne name in the ARcidians, has be n wrongfully r o·arded as a part of the true vascular systen1, and the orgnns by which it is placed in communication with the exterior have been described as " hearts." There are sometimes two and s01netimes four of these "pseudo-hearts" situated in that part of the body wall which helps to bound tho pallial chamber. Each p eudo-heart is divided into a narrow, elongated, external portion (the ~ocalled "ventricle"), which communicates, as Mr. IIancock has proved, by a small apical aperture with the pallial cavity; and a broad, funnel-shaped inner division (the so-ealled "auricle"), co1n1nunicating, on the one hand, by a constricted neck with tho so-eallod " ventricle;" and, on the other, by a wide, patent mouth, with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the pallial lobes. These have been described as parts of the blood vascular system; and arterial trnnks, which have no existence, have been imagined to connect the apices of the ventricles with vascular networks of a similarly mythical character, supposed to open into the branched diverticula. In fact, as Mr. Hancock has so well shown in his splendid aud exhaustive men1oir, published in the Philosophical Transactions for 1857, the true vascular systen1 is completely distinct from this remarkable series of "atrial" chatubers and canals, the function of which would appear to be to convey away excretory matters and the products of the reproductive urgans, which are developed in various parts of the walls of the atrial system. The· precise characters of the true vascular system of the Brachiopoda probably require Rtill further elaboration than they have yet received; and the sarne 1nay be said, notwithstanding the valuable contributions of F. Muller and of Lacaze Duthiers, of their development; but tho shell, the pallial lobes, the in~ testine, and the nervous and the atrial systems, affonl characters amply sufficient to define the class. The next great division is that of the AsoiDIOIDA, which, like tho Brachiopoda, are marino animals, and are very common all ovor tho world; tho more ordinary fonns of thmn being always / |
