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 ECHINODERMATA 621 poles; there are five ambulacra, and the ambulacral plates are about the central cavity. The evolution of this cavity large, simple, and alternating, each being pierced by two podial pores which lie in a small oval depression ; the ambulacrals next into a gut is foreshadowed in some Coelentera by the the mouth form a closed ring of ten plates ; the interambulacrals elliptical shape of the aperture and by the development lie in single columns between the ambulacra, and are separated at its ends of a ciliated channel along which food is from the mouth-area by the proximal ambulacrals just mentioned, swept; we have only to suppose the approximation of the and sometimes by the second set of ambulacrals also; the sides of the ellipse and their eventual fusion, to complete ambulacra end in the five oculars or terminals, which meet in a ring around the anal area and have no podial pores, but one of the transformation of the radially symmetrical Coelenterate them serves as a madreporite ; within this ring is a star-shaped into a bilaterally symmetrical Coelomate with mouth and area filled with minute irregular plates, none of which can safely anus at opposite ends of the long axis. We further suppose be selected as the homologues of the so-called basals or genitals of later forms ; within the ring of ambulacrals around the mouth are that of the four coelomic pouches one was in front of five somewhat pointed plates, which Jaekel regards as teeth, but the mouth, one behind the anus, and one on each side. which can scarcely be homologous with the interradially placed Such an animal, if it ever existed, probably lived near teeth of later echinoids, since they are radial in position ; small the surface of the sea, and even here it may have spines are present, especially around the podial pores. The changed its medusoid mode of locomotion for one in the position of the pores near the centre of the ambulacrals in Bothriocidaris need not be regarded as primitive, since other early direction of its mouth. Thus the bilateral symmetry Palaeozoic genera, not to mention the young of living forms, show would have been accentuated, and the organism shaped that the podia originally passed out between the plates, and were more definitely into three segments, namely, (1) a preoral only gradually surrounded by their substance ; thus the original segment or lobe, containing the anterior coelomic cavity; structure of the echinoid ambulacra differed from that of the early asteroid in the position of the radial vessels and nerves, which (2) a middle segment, containing the gut, and the two here lie beneath the plates instead of outside them. To this point middle ccelomic cavities; (3) a posterior segment, conwe shall recur; palaeontology, though it suggests a clue, does not taining the posterior coelomic cavity, which, however, furnish an actual link either between echinoidea and asteroidea, or owing to the backward prolongation of the anus, became between those classes and Pelmatozoa. The argument from embryology leads farther back. First, as divided into two—a right and left posterior coelom. Each already mentioned, it outlines the general features of the of these cavities presumably excreted waste products to the Diplcurula; secondly, it indicates the way in which this free- exterior by a pore. There was probably a nervous area, moving form became fixed, and how its internal organs were modified in consequence; but when we seek, thirdly, for light on with a tuft of cilia, at the anterior end; while, at all events the relations of the classes, we find the features of the adult in forms that remained pelagic, the ciliated nervous tracts coming in so rapidly that such intermediate stages as may have of the rest of the body may be supposed to have become existed are either squeezed out or profoundly modified. The arranged in bands around the body-segments. Such a form difficulty of rearing the larvae in an aquarium towards the close of the metamorphosis may account for the slight information available as this is roughly represented to-day by the Actinotrocha concerning the stages that immediately follow the embryonic. larva of Phoronis, the importance of which has been Another difficulty is due to the fact that the types studied, and brought out by Masterman. But only slight modifications especially the crinoid Antedon, are highly specialized, so that some of are required to produce the Tornaria larva of the Enterothe embryonic features are not really primitive as regards the class, pneusta and other larvae, including the special type that is but only as regards that particular genus. Thus inferences from embryonic development need to be checked by palaeontology, and inferred from the Dipleurula larval stages of recent forms supplemented by comparison of the anatomy of other living genera. to have characterized the ancestor of the Echinoderms. Minute anatomical research has also aided to establish the We cannot enter here into all the details of comparison Pelmatozoic theory by the gradual recognition in other classes of between these larval forms; amid much that is hypofeatures formerly supposed to be confined to Pelmatozoa. Thus the thetical a few homologies are widely accepted, and the the Pelmatozoan ventral groove are now detected in so different a structure as the echinoid ambulacrum, while an aboral ner- preceding account will show the kind of relation that the vous system, the diminished representative of that in crinoids, has Echinoderms bear to other animals, including what are now been traced m all Eleutherozoa except Holothurians. The broader usually regarded as the ancestors of the Chordata (to which theories of modern zoology might seem to have little bearing on the back-boned animals belong), as well as the nature of the Echinoderma, for it is not long since the study of these animals was compared to a landlocked sea undisturbed by such storms as evidence that their study has been, or may be, made to yield. rage around the origin of the Yertebrata. This, however, is no How the hypothetical Dipleunda became an Echinoderm, more the case. The conception of the Dipleurula derives its chief and how the primitive Echinoderms diverged in structure weight from the fact that it is comparable to the early larval forms so as to form the various classes, are questions to which o other primitive coelomate animals, such as Balanoglossus, Ihoronm, Chretognatha, Brachiopoda, and Bryozoa. So too the an answer is attempted in the following paragraphs :— explanation of radial symmetry and torsion of organs as due to a Confining our attention to that form of Dipleurula (Fig. 6) 1 elinatozoic mode of life finds confirmation in many other phyla. which, it is supposed, gave rise to the Echinoderma, we infer Instead of discussing all these questions separately, with the details necessary for an adequate presentation of the argument, we shall the 1 elmatozoic theory. Such a sketch must pass lightly over debatable ground, and must consist largely of suggestions still in need of confirmation ; but if it serves as a frame into which more precise and more detailed statements may be fitted as they come to the ken of the reader, its object will be attained. Evolution of the Echinoderms.—It is reasonable to suppose that the Coelomata—animals in which the bodycavity is divided into a gut passing from mouth to anus Fig. 6.—Diagrammatic reconstruction of Dipleunda. The creature is and a hollow (coelom) surrounding it—were derived from represented crawling’ on the sea-floor, but it may equally well have been a floating animal. The ciliated bands are not drawn. the simpler Coelentera, in which the primitive body-cavity (archenteron) is not so divided, and has only one aperture from embryological data that its special features were as folserving as both mouth and anus. We may, with Sedgwick, low:—The anterior coelomic cavity was wholly or partially suppose the coelom to have originated by the enlargement divided, and from each half a duct led to the exterior, opening at a pore near the middle line of the back. The middle and separation of pouches that pressed outwards from cavities were smaller, and the ducts from them came to unite the archenteron into the thickened body-wall (such with those from the anterior cavities, and no longer opened structures as the genital pouches of some Ccelentera, not directly to the exterior; whether these cavities were already yet shut off from the rest of the cavity), and they would specialized as water-sacs cannot be asserted, but they certainly had become so at a slightly later stage. The posterior cavities were the probably have been four in number and radially disposed largest, but what had become of their original opening to the exterior
 * V-)W sketch the history of the Echinoderms in accordance with