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Show 42 LECTURE III. ON THE CLASSIFICATION OF ANil\1:ALS. ':rHE ECHINODERMATA, SOOLECIDA, ANNELIDA, CRUSTACEA, ARACHNIDA, MYRIAPODA, AND INSECTA. HITHERTO, it has not been a matter of very great difficulty to discover the characters in which the members of the various classes, which have passed under our notice, agree with one another and differ from the members of all other classes. But to-day we shall be met, at the outset of our studies, by a large series of organisms whieh present us with much greater obstacles,- the result, in a great measure, of imperfect knowledge. The first group on the list-the ECHINODERMATA-comprises the star-fishes, sea-urchins, sea-cucumbers, trepangs, and feather-stars-known technically as Asteridea, Echinidea, Holothuridea, Ophiuridea, Crinoidea, &c.,-marine animals which differ vastly in external appearance, though they all, in the adult state, present a more or less definite! y radiate arrangement of some parts of their organization. That which most remarkably distinguishes the Echinodermata is the nature of the embryo, and the strange character of the process by which the adult form is originated by a secondary development within that embryo. In the great majority* of the Echinoderrnata, the develop- * In Ophiolep1:s squamata and Echinaster sepositus, the larva appears to attain only a very imperfect state of development before the appearance of tho echinoderm body; and careful re-examination is required to decide how far tho larvro of these animals arc truly bilaterally symmetrical. 'l'HE ECHINODERMA'fA. 43 ment of which has been examined, the impregnated eO'g g1ve rise to a froe-swiinining, ovoid, ciliated embryo, the ilia of which soon become restrieted to, and, at the same time, larg ly developed upon, one, two, or more bands, ·which are disposed either transversely, or more or less obliquely to the longitudinal axis of the body, but which are, in any case, bilaterally symmetrical (Fig. 17). Fig. 17 .-Diagram exhibiting the g ncral plan of the development of the Echinoderms (after Miiller).-A. Common form whence the Holothurid (B, B') and Ophiuritl. or Echinid (C, 0') larvro aretl.erived. D, D'. Younger and more advanced stages of the Asterid (Bt"pinnm·ia) larvro. a'. Mouth. b. Stomach. c, Intestine. d, Anus. e. Ciliateu band. e'. Second or anterior ciliated circlet. The parts of the body which carry the ciliated band, or bands, often become developed into processes, which correspond upon each side of the body, and thus render its bilateral ymmetry more marked (Fig. 17, C', D'). And, in t]w larvro of son1e Echinidea and Ophiuridea, othoi· bilaterally symmetrical processes are developed from parts of the body which do not lie in tho course of th :1 eiliated bands. |