620
ECHINODERMATA
Such explanation is an integral part of the Pelmatozoic regarded as primitive. Some of the more striking of these features features. but is provided by no other. are coniined to Synaptidae ; in that family too the absence of the theory, The evidence for the Pelmatozoic theory is supplied by palaeonradial water-vessels from the adult is corretology, embryology, the comparative anatomy of the classes, and a lated with continuity of the circular muscleconsideration of other phyla. Palaeontology, so far as it goes, is a layer while the gut runs almost straight sure guide, but some of the oldest fossiliferous rocks yield remains from’the anterior mouth to the posterior of distinctly differentiated crinoids, asteroids, and echinoids, so anus. Early in the life-history of Synapta that the problem is not solved merely by collecting fossils. Two occurs a stage with live tentacles around the lines of argument appear fruitful. First, a comparison of the mouth, and into these pass canals from the relative numbers of the representatives of the various classes at water-ring, the radial canals to the bodydifferent epochs ; according to this they may be placed in the wall making a subsequent, and only temfollowing order, with the oldest first: Cystidea, Crinoidea, porary, appearance (Fig. 4). Semon called Blastoidea, Asteroidea, Ophiuroidea, Echinoidea. As for Holothis stage the Pentadula, and supposed that, thuroidea, the fossil evidence allows us to say no more than that in its early history, the class had passed — class existed in early Carboniferous times, if not before. The through a similar stage, which he called the fig. 4.—The Pentactula the second method is to work out by slow and sure steps the lines of Pentactcea, and regarded as the ancestor of stage in the development of Synapta. T, of the different families, orders, and classes, and so either all Echinoderms. It has since been proved the five interradial ten- descent arrive at the ancestral form of each class, or to plot out the curve that the five tentacles with their canals are tacles; M, the water- to leading by the of evolution, which may then legitimately be projected into “the interradial, so that one can scarcely look pore, stc to the dark backward and abysm of time.” In this way the many highly on the Pentactula as a primitive stage, stone-canal water-ring,from which modified orders of Cystidea may be traced back to a simple, manywhile the apparent simplicity of the Synap- hangs a Polian vesicle plated ancestor with little or no radiate symmetry (see below). oc, supposed ototidse, at least as compared with other holo- pb; m, longitudinal All the complicated structures of Blastoidea are evolved from a thurians, is now believed to he the result of cysts; muscles; sfc,calcareous fairly simple type, which in its turn is linked on to one of the regressive changes. The Pentactcea, at all spicules; st, stomach. cystid orders. That the crinoids are all deducible from some such events as it sprang from the brain of Semon, (After Semon.) X 24 simple form as that above described under the head ‘ ‘ calycinal must pass to the limbo of mythological an- diameters. theory,” is now generally admitted. Although, in the extreme cestors of the radial food-grooves, nerves, water-vessels, and so Pelmatozoic Theory.—The rejection of the calycinal and Pen- correlation with a radiate symmetry of the theca, such a type differs tactcea theories need not scatter our conceptions of Echmoderm forth, the Cystidea, while in the possession of jointed processes from structure back into the chaos from which they seemed to have from radial plates, bearing the grooves and the various bodyemerged. The idea of a calyculate ancestor, though by no means the outwards from the theca, it differs from. all other connoting fixation, turned men’s minds in the direction of the systems nevertheless ancient forms are known which, if they fixed forms, simply because in them the calyx was best devel- Echinoderms, not themselves the actual links, suggest how the crinoid type oped. The Pentactcea again suggested a search for some primi- are have been evolved from some of the more regular cystids. tive type in which quinqueradiate symmetry was exhibited in may fourth class of Pelmatozoa—the Edrioasteroidea differs fiom circumoral appendages, but had not affected the nervous, The the others the structure of its ambulacra. As in all Pelmatozoa, water-vascular, muscular, or skeletal systems to any great these seemin to have borne ciliated food-grooves protected by extent, and the generative organs not at all. Study ot the movable covering-plates (Fig. 11). Beneath each food-groove was a earliest larval stages has always led to the conclusion that the radial water-vessel and probably nerve and blood-vessel, all which Echinoderms must have descended from some freely-moving structures passed either betweenacertain arranged thecal form with a bilateral symmetry, and, connecting this with the plates, or along a furrow floored by thoseregularly which were then ideas just mentioned, we reach the conception that this sup- in two alternating series. The important plates, and distinctive feature posed bilateral ancestor (or Dipleurula) may have become fixed, is the presence of pores between the flooring-plates, on either and may have gradually acquired a radial symmetry in con- side of the groove; and these, we cannot doubt, served for the sequence of its sedentary mode of life. The different extent o of podia. Thus in a highly developed edrioasteroid, such quinqueradiate symmetry in the different- classes would thus passage Edrioaster itself (Fig. 11), there was a true ambulacrum appardepend on the period at which they diverged from the sedentary as constructed like that of a starfish, but differing m the posstock. The tracing of this history, and the explanation of the ently of a ciliated food-groove protected by covering-plates. Ihe general characters of Echinoderms and of the differentiating session forms of Edrioasteroidea, with their more sac-like body features of the classes in accordance therewith, constitutes the simpler undifferentiated plates, may well have been derived from early Pelmatozoic theory. ,, , „ , . , ,, and Cystidea of yet simpler structure, and there seems no reason to The word “Pelmatozoa” literally means stalked animals, follow Jaekel in regarding the class as itself the more primitive. but the name is now used to denote all Cystidea, Blastoidea, Turning to fossil Asteroidea, we find the earlier ophiunds scarcely Crinoidea, and Edrioasteroidea, as opposed to the other classes, distinguishable from the asterids, while in the alternation of the which may be called Eleutherozoa. Many Pelmatozoa have, it is ambulacrals, which undoubtedly correspond to the flooring-plates true, no stalk, while some are freely-moving, but all agree in the of Edrioastcr, both groups approach the Pelmatozoan type. Ihese possession of certain characters obviously connected with a hxed facts have been expressed by in his names Enciinasterise and mode of life. Thus, the mouth is central and turned away from Ophio-encrinasterise. There Stiirtz is no difficulty in deducing the highly the sea-floor ; the animal does not seize its food by tentacles, limbs differentiated asterids and ophiurids a later day from these or jaws, neither does it move in search of it, but a series of ciliated simpler types. The evolution of theofmodern Echinoidea from grooves which radiate from the mouth sweep along currents o Palseozoic ancestors is also well understood, but in this case water, in the eddies of which minute food-particles are caught up their ancestral form to which the palaeontologist is led does not at and carried down into the gullet; the undigested food is driven the sight present many resemblances to the Pelmatozoa. it is, out through an anus which is on the upper or oral side ot the first characterized by simplicity of structure, and a short theca, but as far distant as practicable from the mouth and ciliated however, of it will serve to clear the problem from unnecessary grooves. Such characters are found in any primitive, sedentary description (Fig. 5), a small echmoid from the group. More peculiarly Echinoderm features, in which the difficulties. Bothriocidaris anus.. Pelmatozoan nature is manifest, are the enclosing of the viscera in a calcified and plated theca, for protection against those inter_ enemies from which a fixed animal cannot flee ; the development, at a m bula crum/P'^ the aboral pole of this theca, of a motor nerve-centre giving off branches to the stroma connecting the various plates of the theca and of its brachial, anal, and columnar extensions, and thus termenafrco-ordinating the movements of the whole skeleton ; the absence of ambulacru suckers from the podia, which, when present, are respiratory, not locomotor, in function. There are other features of most, it not 'pore pair all Pelmatozoa that appear to be due to a fixed existence, but 'anus those are also found in the Eleutherozoa. The Pelmatozoic theory ' tu f/ercies for spines, mouth thus regards the Pelmatozoa as the more ancestral forms, and R the Pelmatozoan stage as one that must have been passed A through by all Echinoderms during their evolution from the Fig 5 —Bothriocidaris globulus. A, from the side ; B, the plates around Dipleurula. It might be possible to prove the origin of all the IborS pole. (After Jaekel) The short spines which were attached to the tubercles are not drawn. classes from Pelmatozoa, without thereby explaining the origin of such fundamental features as radial syinmetry, the developmental Ordovician rocks of Esthonia, is in essential structure just the metamorphosis, and the torsion that affects both gut and body- form demanded by comparative palaeontology to make a starting cavities during that process ; but the acceptance of a Dipleurula point. It is spheroidal, with the mouth and anus at IP as the common ancestor necessitates an explanation of these
