Francis Arthur Bather

“ The problem of the interrelations of the classes will thus be reduced to its simplest terms, and even questions as to the nature of the primitive Echinoderm and its affinity to the ancestors of other phyla may become more than exercises for the ingenuity of youth. Work has been and is being done by the laborious methods here alluded to, and though the diversity of opinion as to the broader groupings of classification is still restricted only by the number of writers, we can point to an ever-increasing body of assured knowledge on which all are agreed. ”

“ Evolution of the Echinoderms.—It is reasonable to suppose that the Coelomata—animals in which the body-cavity is divided into a gut passing from mouth to anus and a hollow (coelom) surrounding it—were derived from the simpler Coelentera, in which the primitive body-cavity (archenteron) is not so divided, and has only one aperture serving as both mouth and anus. ”

“ There is no reason to suppose that the central apical plate of certain free-swimming crinoids has any more to do with the distal foot-plate of the larval Antedon stem than has the so-called centro-dorsal of Antedon itself, which is nothing but the compressed proximal end of the stem. As for the supposed basals of Echinoidea, Asteroidea and Ophiuroidea, they are scarcely to be distinguished among the ten or more small plates that surround the anus of Bothriocidaris, which is the oldest and probably the most ancestral of fossil sea-urchins (fig. 5) . ”

“ Although, in the extreme correlation of the radial food-grooves, nerves, water-vessels, and so forth, with a radiate symmetry of the theca, such a type differs from the Cystidea, while in the possession of jointed processes from the radial plates, bearing the grooves and the various body-systems outwards from the theca, it differs from all other Echinoderms, nevertheless ancient forms are known which, if they are not themselves the actual links, suggest how the crinoid type may have been evolved from some of the more regular cystids. ”

“ There was also sufficient comprehension of the differences between the main classes of Echinoderms—the sea-urchins or Echinoidea, the starfish or Asteroidea, the brittle-stars and their allies known as Ophiuroidea, the worm-like Holothurians, the feather-stars and sea-lilies called Crinoidea, with their extinct relatives the sac-like Cystidea, the bud-formed Blastoidea, and the flattened Edrioasteroidea—while within the larger of these classes, such as Echinoidea and Crinoidea, fair working classifications had been established. ”

“ Food is not conveyed by a subvective system of ciliated grooves, but is taken in directly by the mouth. The anus when present is typically aboral, and approaches the mouth only in a few specialized forms. The aboral nervous system, if indeed it be present at all, is very slightly developed. The circumoesophageal water-ring may lose its connexion with the exterior medium; the podia (absent only in some exceptional forms) may be locomotor, respiratory or sensory in function, but usually are locomotor tube-feet. ”

“ For, whereas to one brilliant suggestion of far-reaching homology another can always be opposed, by the detailed comparison of individual growth-stages in carefully selected series of fossils, and by the minute application to these of the principle that individual history repeats race history, it actually is possible to unfold lines of descent that do not admit of doubt. ”

“ Each plate of the upper circlet supports an arm, and is called a “radial”; the plates of the lower circlet, the “basals,” rest on the stem and alternate with those of the upper circlet, i.e. are interradial in position. Some crinoids have yet another circlet below these, the constituent plates of which are called “infrabasals,” and are situated radially. The tegmen in most primitive forms, as well as in the embryonic stages of the living Antedon (fig. 2) , consists of five large triangular plates, alternating with the radials, and called “orals,” because they roof over the mouth. ”

“ Study of the earliest larval stages has always led to the conclusion that the Echinoderms must have descended from some freely-moving form with a bilateral symmetry, and, connecting this with the ideas just mentioned, we reach the conception that this supposed bilateral ancestor (or Dipleurula) may have become fixed, and may have gradually acquired a radial symmetry in consequence of its sedentary mode of life. ”

“ The food-grooves are exothecal, i.e. are stretched out from the theca on jointed skeletal processes (brachioles) . These either are close to the mouth or are removed from it upon a series of ambulacral or sub-ambulacral plates not derived immediately from thecal plates, or are separated from the oral centre by hypothecal passages passing beneath terminal plates. The stereom and stroma become arranged in folds and strands at right angles to the sutures of the thecal plates; in higher forms the stereom-folds are in part specialized as pectini-rhombs. Families: Echinosphaeridae ”

“ The evidence for the Pelmatozoic theory is supplied by palaeontology, embryology, the comparative anatomy of the classes, and a consideration of other phyla. Palaeontology, so far as it goes, is a sure guide, but some of the oldest fossiliferous rocks yield remains of distinctly differentiated crinoids, asteroids and echinoids, so that the problem is not solved merely by collecting fossils. ”

“ The thecal plates, however irregular in some species, always show defined basals and a distinct plate (“radial”) at the end of each ambulacrum; they are in all cases so far affected by pentamerous symmetry that their sutures never cross the ambulacra. ”

“ Pentactaea Theory.—In opposition to the calycinal theory has been the Pentactaea theory of R. Semon. There have always been many zoologists prepared to ascribe an ancestral character to the holothurians. The absence of an apical system of plates; the fact that radial symmetry has not affected the generative organs, as it has in all other recent classes; the well-developed muscles of the body-wall, supposed to be directly inherited from some worm-like ancestor ”

“ Analysis of Echinoderm Characters.—Regarding 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 coelomate 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 coelomic cavities are primitively transformed into two hydrocoels communicating with the exterior indirectly through a duct or ducts of the anterior coelom ”

“ All living, and most extinct, Echinoderms show the following features, almost certainly due to an ancestral pelmatozoic stage:—An incomplete radial symmetry, of which five is usually the dominant number, is superimposed on the secondary bilateralism, owing to the outgrowth from the mouth region of one unpaired and two paired ciliated grooves; these have a floor of nervous epithelium, and are accompanied by subjacent radial canals from the water-ring, giving off lateral podia and thus forming ambulacra, and by a perihaemal system of canals apparently growing out from coelomic cavities. ”

“ All living Echinoderms have a lacunar, haemal system of diverse origin; this, the ambulacral system, and the coelomic cavities, contain a fluid holding albumen in solution and carrying numerous amoebocytes, which are developed in special lymph-glands and are capable of wandering through all tissues. ”

“ 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 a more difficult problem, on which Bothriocidaris sheds no light. An Upper Silurian echinoid, however, Palaeodiscus, is believed by W. J. Sollas and W. K. Spencer to have had in its ambulacra an inner as well as an outer series of plates. ”

“ Cryptozonia, with marginals inconspicuous and somewhat atrophied in the adult, the supero-marginals separated from the infero-marginals by intercalated plates, with papulae distributed over the whole body, with ambulacrals crowded and narrow, either ambulacrals or adambulacrals prominent in the mouth-skeleton, with pedicellariae stalked or sessile. ”

“ Class II. Stelliformia (=Asteroidea sensu lato) .—Eleutherozoa with a depressed stellate body composed of a central disk, whence radiate five or more rays; this radiate symmetry affects all the systems of organs, including the genital. The radial water-vessels lie in grooves on the ventral side of flooring-plates (usually called “ambulacrals”) ; they and their podia are limited to the oral surface of the body and their extremities are separated from the apical plates by a stretch of dorsal integument containing skeletal elements ”

“ Rays simple or branched, capable of coiling, since the vertebrae articulate by surfaces of hour-glass shape; ventral arm-plates, and often the others, much reduced; spines reduced or absent. Families: Euryalidae, Gorgonocephalidae, Astrochelidae, Astroschemidae, Astronycidae.
The Silurian genera Eucladia and Euthemon have the rays greatly reduced and merged in the disk, so that the ambulacrals are unseen. There are a few large dorsal, lateral and ventral arm-plates, and at the angles of the latter emerge huge podia with a granular or plated skin. ”

“ Flexibilia.—Dicyclica in which proximal brachials are incorporated in the dorsal cup, either by their own sides, or by interbrachials, or by a finely plated skin, but never rigidly; plates may occur between RR. Tegmen flexible, with distinct ambulacrals and numerous small interambulacrals; mouth and food-grooves remain supra-tegminal and open. Top columnal a persistent proximale, often fusing with IBB, which are frequently atrophied in the adult. ”

“ If this be correct, the only change from Edrioaster, as regards the ambulacra, was that in Palaeodiscus the covering-plates could no longer open, but closed permanently over the whole groove, while the podia issued through slits between them. In more typical echinoids the covering-plates alone remained to form the ordinary ambulacral plates, while the flooring-plates disappeared, the canals and other organs remaining as before. ”

“ Holectypoida.—Ambulacrals sometimes compound, with one or two pores to a plate, some dorsal podia begin to assume respiratory function; interambulacrals multi-tuberculate, none resorbed; mouth central, with jaws weak or wanting, with external gills and sphaeridia; anus exocyclic. Families: Pygasteridae, Discoidiidae, Galeritidae, Conoclypeidae ”

“ Class II. Blastoidea.—Pelmatozoa in which five (by atrophy four) epithecal ciliated grooves, lying on a lancet-shaped plate (? always) , radiate from a central peristome between five interradial deltoid plates, and are edged by alternating side-plates bearing brachioles, to which side-branches pass from the grooves. Grooves and peristome protected by small plates, which can open over the grooves. The generative organs and coelom probably did not send extensions along the rays into the brachioles ”

“ Grade A. Palophiurae.—Ambulacrals not yet forming complete vertebrae; plates of disk not yet specialized into mouth, radial or genital shields.
Stage a. Allostichia (= Lysophiurae) .—Ambulacrals alternating and unfused, groove uncovered by ventral arm-plates. Families: † Protasteridae, † Protophiuridae.
Stage b. Zygostichia.—Ambulacrals opposite and, except in Ophiurinidae, fused; ventral arm-plates developed in some. Families: † Ophiurinidae, † Lapworthuridae, † Furcasteridae, † Palastropectinidae, † Eoluididae, † Palaeophiomyxidae. ”

“ Batocrinoidea.—RR separated by a heptagonal anal; first brachial usually quadrangular. Families: Tanaocrinidae, Xenocrinidae, Carpocrinidae, Barrandeocrinidae, Coelocrinidae, Batocrinidae, Periechocrinidae. ”