OCR Text |
Show 46 ON CLASSIFICATION. with the exterior is doubtful, and still more so in Crinoids. In Holothurids no such c01nmunication obtains, the madrcporic canals and their tubercles depending freely from the circular canal into the perivisceral cavity. Whether the larva possessed a skeleton or not, the adult Echinoderm presents a calcareous framework which is developed quite independently of· that of the larva. This skeleton may be com.posed of mere detached spicula, or plates, as in tho Holothurids; or of definitely disposed ossicula, or regular plates, as in other Echinoderms. In the latter case its parts are always disposed with a certain reference to the disposition of the ambulacral system, and hence have a 1norc or less distinctly radiate arrangement. It might be expected, in fact, that the arrangmnent of the organs of support should follow more or less closely that of the chief organs of movement of the adult Echinoderm, and it is not surprising to find the nervous system similarly related. It is, in all adult Echinoderms, a ring-like, or polygonal, ganglionated cord, situated superficially to that part of the ambulacral system which surrounds the mouth, and sending prolongations parallel with, and superficial to, the radiating ambulacral trunks. The reproductive organs of the Echinoderms, which usually open upon, or between, parts of the radially disposed skeleton, commonly partake of the radial symmetry of that skeleton; but they have no such radial symmetry in the Holothurridea. The alimentary canal of the adult Echinoderm is still less dependent upon the skeleton, and only in one group, tho Asteridea, exhibits anything approaching a radiate disposition. Where skeletal elements are developed around tho mouth or gullet, however, they have a radial disposition; a , e. g., tho parts of the so-called "lantern of Aristotle." The vascular system which exists in many, if not all, adult Echinoderms, but the true nature of which is by no means understood at present, is closely related both to the alimentary and to the ambulacral systems, and partakes of the disposition of both. No Echinoderm whatsoever has its organs, internal or external, disposed with that absolute and perfect radial symmetry THE SCOLECIDA. 47 which is exhibited by a Medusa, the tendency towards that ldnd of symmetry being always disturbed, either by the disposition of the alimentary canal, or by that of some part of the ambulacral apparatus. Very often, as in the Spatangoid sea-urchins, and in many Holothtwidea, the ambulacral and nervous systmns alone exhibit traces of a radial arrangement; and in the larval state, as we have seen, radial symmetry is totally absent, the young Echinoderm exhibiting as complete a bilateral symmetry as Annelids, or Insects. Nothing can be more definite, it appears to me, than the class Echinodermata, the leading characteristics of which have just been enumerated; but it is a very difficult matter to say whether the seven groups, son1e of considerable extent, which are massed under the next head, that of ScoLECIDA, are rightly associated into one class, or should be divided into several. The seven groups to which I refer are the Rotijera (or vVheelanimalcules), the Tttflrbellaria, the Trematoda (or flukes), the Tmniada (or tapeworms), the Nematoidea (or threadworms), the Acanthocephala, and the Gordiacea. Of these, five are composed of animals parasitic upon others; and exhibiting the anomalies of structure and of development which might be expected from creatures living under such exceptional conditions. There is one peculiarity of organic structure which the first four of these groups certainly have in common; they all present what is termed the " water-vascular system," -a remarkable set of vessels which communicate with the exterior by means of one, or Inore, apertures situated upon tho surface of the body, and branch out, mO:re or less extensively, into its substance. In the Rot~fera the external aperture of the water-vascular system is single, and situated at the hinder end of the body; it communicates with a large, rhythmically contractile, sac, whence two trunks proceed, which usually give off short lateral branches, and terminate in the ciliated "trochal disk " of the Rotifer, in the middle of which its mouth is placed. Both the · lateral offshoots and the terminal branches contain vibratile cilia. The Trematode and Trenioid worms have a similar, but usnally much ~ore ramified apparatus; and it is interesting to observe that, 1n these aniinals, as in the Aspido,qaster conchicola |