ECHINODERMATA out along with these grooves, each system starting from a ring around the gullet. At last a quinqueradiate symmetry influenced the plates of the theca, partly through the development of a plate at the end of each groove (terminal), partly through plates at the aboral pole of the theca (basals and infrabasals) arising in response to mechanical pressure, but soon intimately connected with the cords of an aboral nervous system. Before the latter plates arose, the stem had developed by the elongation and constriction of the fixed end of the theca, the gradual regularization of the plates involved, and their coalescence into rings. The crinoid type was differentiated by the extension of the food-grooves and associated organs along radial outgrowths from the theca itself. These constituted the arms (brachia), and five definite radial plates of the theca were specialized for their support. These radials may be homologous with the terminals already mentioned, but this is neither necessary nor certain. In this development of brachial extensions of the theca the genital organs were involved, and their ripe products formed at the ends of the brachia or in the branches therefrom. The remains of the original genital gland within the theca became the “axial organ” surrounded by the ‘ ‘ axial sinus ” derived from the anterior coelom, and this again by structures derived from the right posterior coelom, which, as explained above, had been depressed to the aboral pole. These last structures formed a nervous sheath around the axial sinus with its bloodvessels, and became divided into five lobes correlated with the five basals (the “chambered organ”) and forming the aboral nervecentre. Before these changes were complete the Holothurioidea must have diverged, by the assumption of a crawling existence. Thus in them the mouth and anus reverted to opposite poles, and only the torsion of the gut and coelom, and the radial extensions of the nervous, water-vascular, and blood-vascular systems, testified to their Pelmatozoan ancestry. The ciliated grooves, no longer needed for the collection of food, closed over, and are still traceable as ciliated canals overlying the radial nerves. At the same time the thecal plates degenerated into spicules. The Edrioasteroidea followed a different line to that of the cystids above mentioned and their descendants. The theca became sessile, and in its later developments much flattened (Fig. 11). Mouth, water-pore, and covering lobes of fleorflooring plates plates ible integument
covering plates 'hydropore frame' Fig. 11.—Edrioaster. A, upper or oral surface of E. Bigsbyi, with the covering-plates on the anterior and left posterior food-grooves, hut removed from the others, which show only the flooring-plates, between which are pores; B, under surface of E. Buchianus, with coveringplates on right posterior and right anterior food-grooves (left hand in the drawing). The if-- denotes the position of the anal interradius. anus remained as in Aristocystis, but the five ciliated grooves radiated from the mouth between the thecal plates rather than over them, and were, as usual, protected by covering-plates. The important feature was the extension of radial canals from the watersac along these grooves, with branches passing between the flooringplates of the grooves (Fig. 12, A). The resemblance of the flooringplates to the ambulacral ossicles of a starfish is so exact that one can explain it only by supposing similar relations of the watercanals and their branches (podia). On the thinly-plated under surface of well-preserved specimens of Edrioaster are seen five interradial swellings (Fig. 11, B). These are likely to have been produced by the ripe genital glands, which may have extruded their products directly through the membranous integument of the under side. No other way out for them is apparent, and it is clear that Edrioaster was not permanently and solidly fixed to the sea-floor. Now comes a great change, unfortunately difficult to follow whether in the fossils or in the modern embryos. We suppose some such form as Edrioaster, which appears to have lived near the shore, to have been repeatedly overturned by waves. Those that were able to accommodate themselves to this topsy-turvy existence, by taking food in directly through the mouth, survived, and their podia gradually specialized as sucking feet. Such a form as this, when once its covering-plates had atrophied, would be a starfish without more ado (Fig. 12, B); but the sea-urchins present
623
a more difficult problem. We may suppose that the radial watercanals sank beneath the flooring-plates of the grooves, and that the covering-plates disappeared, in which case the plates that in the echinoid are pierced by podia would be homologous with the ambulacral ossicles of an asteroid and the flooring-plates of Edrioaster. Or we may suppose the canals and other organs to have remained as they were, the covering-plates to have closed permanently over them, merely leaving slits for the podia between them, and the flooring-plates to have disappeared. In either case we have to admit a closure of the integument over the ciliated groove (Fig. 12, D, e) just as in holothurians, since this is necessitated by
A. Edrioasteroid. covering pla.tr
B. Asteroid. ambulacral plate cuter neree
ccelomic space. y' -. ' • .—
deeper nerve water vessel
coelom. C. Crinoid. D. Echinoid. Fio. 12.—Diagrammatic sections across the ambulacra of A, C, Pelmatozoa, and B, D, Eleutherozoa, placed in the same position for comparison. S, blood-spaces, of which the homology is still uncertain. anatomical evidence. The genital organs in both Asteroidea and Echinoidea would retain the interradial position they first assumed in Edrioaster; and in Echinoidea their primitive temporary openings to the exterior were converted into definite pores, correlated ■with five interradially placed plates at the aboral pole. The anus also naturally moved to this superior and aboral position. In the Echinoidea the water-canals and associated structures, ending in the terminal plates, stretched right up to these genital plates ; but in the Asteroidea they never reached the aboral surface, so that the terminals have always been separated from the aboral pole by a number of plates. Analysis of Echinoderm Characters.—To follow the further evolution of the several classes would require a series of independent articles; reference must perforce be made to works {vide infra) in which this has recently been attempted. Kegarding the Echinoderms as a whole in the light of the foregoing account, we may give the following analytic summary of the characters that distinguish them from other ccelomate animals :— They live in salt or brackish water ; a primitive bilateral symmetry is still manifest in the right and left divisions of the coelom ; the middle ccelomic cavities are primitively transformed into two hydrocoels communicating with the exterior indirectly through a duct or ducts of the anterior coelom ; stereom, composed of crystalline carbonate of lime is, with few exceptions, deposited by special amoebocytes in the meshes of a mesodermal stroma, chiefly in the integument; reproductive cells are derived from the endothelium, apparently of the anterior coelom ; total segmentation of the ovum produces a coeloblastula and gastrula by invagination; mesenchyme is formed in the segmentation cavity by migration of cells, chiefly from the hypoblast. Known Echinoderms show the following features, imagined to be due to an ancestral pelmatozoic stage:— Increase in the ccelomic cavities of the left side, and atrophy of those on the right; the dextral coil of the gut, recognizable in all classes, though often obscured ; an incomplete secondary bilateralism about the plane including the main axis and the water-pore or its successor, the madreporite, often obscured by one or other of various tertiary bilateralisms ; the development of the hydrocoel into a circumoral, arcuate or ring canal ; development through a free-swimming, bilaterally symmetrical, ciliated larva, of which in
