OCR Text |
Show 40 ON CLASSIFICA'riON. fundamental characters. ~rhus, the mantle is related to the body, as iu Pteropoda and Gasteropoda; when an external shell exists it is composed of a single piece, and the Cephalopods have Fig. 16. Fig. 16.-Diagrammatic section of a female Cephalopod (Sepia o.fficinab"s). a, Buccal ma~s sm:rounded by the lips, and showing the horny jaws and tongue; ~' msophagus; c, mit vary gland; d, stomaeh; e, pyloric crecum ; f, the funnel; u, the wtestme; It, tl~e anus ; i, the ink-hag; ll, the place of the systemic heart; z, the liver ; n, the hep~tH.: duct of the left side; o, the ovary; p, the oviduct; q, one of the apertures by w~uch the atrial sy~tem, or water-chambers, are placed in communication with the extenor; r, one of the bmnchim; s, the principal ganglia aggregated round the msophagus; m, t~e mantle; sh, the internal shell, or cuttle-bone. 1, 2, 3, 4, .5, the produced and modi· fied margins of the foot, constituting the so-called "arms" of the Sepirt. an odontophore constructed upon just the same principle a8 that of the other classes. The nervous system, the foot, and the epipodia exhibit the same primary relations as in these groups, and there is a distinet head, with ordinarily well- 'l'HE 'EPIL\.LOPODA. 41 dev loped optic and olfactory organ . That \vhich e sentially thara ·tori es the Cephalopoda, in fact, is simply the manner iu which, in the course of development (as IColliker long since proved), the 1nargins of the foot prop r and the opipodia become modified and change their relations. The margins of the foot arc produced into more or 1 ss numerous tentacular appenclng ·, often provided with ingularly constructed sucker or ac tabula ; and the antero-lateral parts of each sid of the foot xt nd forwards beyond the head, uniting with it and ·with one another; so that, at length, the mouth, from having been ituated, a usual, above the anterior margin of the foot, come to b plac d in the midst of it. The two epipodia, on the other hand, unit posteriorly above the foot, and where they coale ce, give ri o either to a folded muscular expansion, the edges of which ar \ simply in apposition, as in Nautilus; or to an elongated flexible tube, the apex of which projects beyond the margin of the mantle (Fig. 16, f), called the "funnel" or "infundibulum," as in the dibranchiate Cephalopoda. The Cephalopoda present a vast nu1nber of the most int \_ resting features, to which it would be necessary to devote much attention if we were studying all the organic peculiarities manifested by tho class ; but it is in the characters of foot and of the epipodium that the definition of the class mu t he chi fly ought. In addition, the flexure of the intestine is, in all Cephalopods, neural; and the mouth is always provided with a horny or more or less calcified beak, like that of a parrot, composed of two curved pieces, which mov in the median antero-posterior plane of the body; and one of which, that on the neural side, is alway longer than the other. |