622
ECHINODERMATA
two; one close to the mouth is almost certainly the waterpore, while that nearer the anus is regarded as a genital aperture. Which of the coelomic cavities this last is connected with is uncertain, for there is considerable doubt as to the origin of the genital glands in the embryonic development of recent echinoderms. It seems clear, however, that there was but a single duct and a single bunch of reproductive cells, as in the holothurians, though perhaps bifurcate, as in some of those animals. The line between mouth and anus, along which these openings are situate, corresponds with the plane of union between the two horns of the curved left posterior coelom, the united walls of which form the “dorsal mesentery.” Since this must have, on our theory, enclosed the parietal canal from the anterior coelom, it is possible that the genital products were developed from the lining cells of that cavity, and that the genital pore was nothing but its original pore not yet united with that from the water-sac. The concrescence of these pores can be traced in other cystids ; but as the genital organs became affected by radial symmetry the original function of the duct was lost, and the reproductive elements escaped to the exterior in another way. Aristocystis may have had ciliated food-grooves leading to its mouth, but these have left no traces on the structure v/cUerpore: of the test. Traces, however, -mouth are perceptible in genera believed to be descended from such a simple type, and the majority may be grouped under two heads. One group includes those in which the grooves wander outwards from the mouth over the thecal plates, which gradually become arranged regularly on either side of the grooves, while further extensions ascend from Fig. 9.—Fungocystis rarissima, one the grooves on small jointed of the Diploporita, in which the plates bordering the foodprocesses called “brachioles” thecal are not yet regularly ar(Fig. 9). In the other group grooves ranged. The brachioles are not the grooves do not tend so much drawn. to stretch over the theca as to ... be raised away from it on relatively larger brachioles, arising close around the mouth (Fig. 10). , , . These two types are, in the main, correlated with two gradual ■parietal yul. differentiations in the minute structure of the thecal plates. Origincanal. ally the calcareous substance of the plates (stereom) was pierced by irregular canals, more or less vertical, and containing strands left of the soft tissue (stroma) that deposited the stereom, as well as right posterior spaces filled with fluid. In the former group (Fig. 9) these posterior coelom.. canals became connected in pairs (diplopores) still perpendicular coelom. to the surface, and this structure, combined with that of the anterior grooves, characterizes coelom. pre-oral lobe the order — Diploremains of porita. In the latter group (Fig. 10) the nerve ganglion canals, that is to say, the stroma - strands, hrctchioles. pI0 7 Diagrammatic reconstruction of primitive Pelmatozoon, seen from came to lie parallel tlie side. The plates of the test are not drawn ; their probable appearto the surface and to ance may be gathered from Fig. 8. the sutures bethe pore came to lie between mouth and anus. The forward portion cross the plates, of the anterior coelom shared in the constriction and elongation of tween were thus more the preoral lobe ; but its hinder portion was dragged up along with which and more the water-pore and formed a canal lying along the outer wall (the flexibly united; since parietal canal). As the gut coiled, it pressed inwards the middle strongly canals crossing pecttnir/iomfr. of the left posterior coelom of the Diplvurulcb, and drew the whole the suture naturally towards the mouth, while the corresponding cavity on the right each occupy a rhombic was jtpressed- down, by* mouth. month. —water pore. the order is the stomach towards the -genitaipore area, called Rhombifera. AAecct. fixed end of the animal anus. At first the grooves and became involved in were three, one prothe elongation of that ceeding from each end region. These changes, of the mouth-slit, and which may still be traced the third in a direcin the development of tion opposed to the Antcdon, resulted in the anus ; with reference primitive Pelmatozoon to the Pelmatozoan (Fig. 7), represented in structure, the anal side the rocks by such a genus may be termed posstem. as Aristocystis (Fig. 8). terior, and this groove The pear-shaped body is anterior. Eventually encased in a theca formed each lateral groove by a number of polyforked, so that there gonal plates, and is attachment. 10.—Chirocrinus -alter, one of the Rhombi were five grooves. Fig.fera, attached by its narrow showing the reduced number and regular gradually im- arrangement of the thecal plates, and the con end. On the broad upper- Fig. 8.—Aristocystis hohemicus', side-view of These pressed themselves on centration surface are four openings, the theca. The internal structure may be the theca and influ- Jaekcl.) of the brachioles. {Adapted from that nearest the centre gathered from Fig. 7. (x J diam.) enced the arrange being the mouth, which . . ,, of the internal organs: it is fairly safe to assu™e ^ is slit-like, and that nearest the periphery being the anus. ment nerves, blood-vessels, and branches from the water-sac stretched The two other openings are minute, and placed between those
is uncertain. The genital products were derived from the lining of the coelomic cavities, but it would not be safe to say that any particular region was as yet specialized for generation. The epithelium of the outer surface was probably ciliated, and a portion of it in the preoral lobe differentiated as a sense-organ, with longer cilia and underlying nerve-centre, from which two nerves ran back below the ventral surface. Into the space between the walls of the coelom and the outer body-wall, originally filled with jelly, definite cells now wandered, chiefly derived from the coelomic walls. Some of these cells produced muscles and connective tissue ; others absorbed and removed waste products, iron salts, calcium carbonate, and the like, and so were ready to be utilized for the deposition of pigment or of skeletal substance. In some of these respects the Dipleurula may have diverged from the ancestor of Enteropneusta and of other animals, but it could not as yet have been recognized as echinodermal by a zoologist, for it presented none of the structural peculiarities of the modern adult echinoderm. Now ensued the great event that originated the phylum—the discovery of the sea-floor. This being apprehended by the sensory anterior end, it was by that end that the Dipleurula attached itself; not, however, by the pole, since that would have interfered at once with the sensory organ, but a little to one side, the right side being the one chosen for a reason we cannot now fathom; it may be that fixation was facilitated by the presence of the pore on that side, and by the utilization of the excretion from it as a cement. The first result was that which is always seen to follow in such cases—the passage of the mouth towards the upper surface (Fig. 7). As it passed up along the left side, the gut caught hold of the left water-sac and pulled it upwards, curving it in the process ; this being attached to the left duct from the anterior bodycavity, this structure with its water-pore was also pulled up, and Slone canal. 7ncu/A. hydropore. yonoporej left hydrocoel.
