Page:EB1911 - Volume 08.djvu/907

 apical plates by a stretch of dorsal integument containing skeletal elements; the opening of the water-vascular system (madreporite) is not connected with a definite apical plate or system of plates.

The starfish, brittle-stars and their allies (see ) have for the last fifty years usually been divided into two classes—Asteroidea and Ophiuroidea, each equivalent to the Holothurioidea or Echinoidea. Recently, however, some authors, e.g. Gregory, have attempted to show that these classes cannot be distinguished. It is true that some specialized forms, such as the Brisingidae among starfish, Astrophiura and Ophioteresis among ophiurans, contravene the usual diagnoses; but this neither obscures their systematic position, nor does it alter the fact that since early Palaeozoic times these two great groups of stellate echinoderms have evolved along separate lines. If then we place these groups in a single class, it is not on account of a few anomalous genera, but because the characters set forth above sharply distinguish them from all other echinoderms, and because we have good reason to believe that the ophiurans did not arise independently but have descended from primitive starfish. For that class Bell’s name Stelliformia is selected since it avoids both confusion and barbarism.

Sub-class I. —Stelliformia in which the ambulacral groove always remains open and the podia serve as tube-feet (fig. 12, B); the rays as a rule pass gradually into the disk, and contain both genital glands and caecal extensions of the digestive system; an anus usually present; respiration is by tubular extensions from the body-cavity (papulae); skeletal appendages, in addition to small spines, are either small grasping organs (pedicellariae), or clumped spines (paxillae), or branched spines bearing a membrane.

No existing classification of the Asterida is satisfactory even for the recent forms, still less when the older fossils are considered. A separation of the latter as Palasterida, because of their alternating ambulacrals, from the recent Euasterida with opposite ambulacrals, is now discarded and an attempt made to arrange the Palasterida in divisions originally established for Euasterida. Those divisions fall under three schemes. C. Viguier has divided the starfish into: Astéries ambulacraires, with plates of ambulacral origin prominent in the mouth-skeleton, pedicellariae stalked, and straight or crossed, podial pores usually quadriserial; Astéries adambulacraires, with adambulacrals prominent in the mouth-skeleton, pedicellariae sessile, and forcipiform or valvular, podial pores usually biserial. Perrier, at first laying greater stress on the nature of the pedicellariae and afterwards on the form of the mouth-skeleton, has gradually perfected a scheme of five orders: (1) Forcipulata, with pedicellariae stalked, and straight or crossed; (2) Spinulosa, with pedicellariae sessile and forcipiform; (3) Velata, with membraniferous spines; (4) Paxillosa, pedicellariae represented by an ossicle of the test and the spines covering it, the whole forming a paxilla; (5) Valvata or Granulosa, with pedicellariae sessile and valvular or salt-cellar shaped. A more widely accepted scheme is that of W. P. Sladen, who divided the Euasterida into two orders; (1) Phanerozonia, with marginals large and highly developed, the supero-marginals and infero-marginals contiguous, with papulae confined to the dorsal surface, with ambulacrals well spaced and usually broad, adambulacrals prominent in the mouth-skeleton, with pedicellariae sessile; (2) 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.

We give here a list of the families separated into Sladen’s orders and grouped under Perrier’s divisions, extinct families being marked †.

1. —Unclassed Famm., † Palaeasteridae, † Palasterinidae, † Taeniasteridae, † Aspidosomatidae. Paxillosa, Luidiidae, Astropectinidae (fig. 16), Archasteridae restr. Verrill, Porcellanasteridae, Chaetasteridae. Valvata, Benthopectinidae, Goniopectinidae, Plutonasteridae, Odontasteridae, Pentagonasteridae, Antheneidae, Pentacerotidae, Gymnasteriidae. Spinulosa, Poraniidae, Asterinidae.

2. —Unclassed Famm., † Sturtzasteridae (= Palaeocomidae Greg.), † Lepidasteridae, † Tropidasteridae. Valvata, Linckiidae restr. Perr. Spinulosa, Echinasteridae, Solasteridae (fig. 17), Korethrasteridae. Velata, † Palasteriscidae, Pterasteridae, Pythonasteridae, Myxasteridae. Forcipulata, Stichasteridae, Zoroasteridae (fig. 3, D), Heliasteridae, Pedicellasteridae, Asteriidae, Brisingidae.

Sub-class II. —Stelliformia in which the ambulacral groove, though open in the oldest forms, soon becomes closed, while the podia cease to serve as tube-feet; the rays as a rule spring abruptly from the disk and contain neither genital glands nor digestive caeca; no anus; respiration may be through clefts at the bases of the rays, but not by papulae; skeletal appendages confined to spines, usually of simple structure.

There is as yet no satisfactory classification of the Ophiurida into orders expressing lines of descent; even as regards families, leading writers are at variance. The following scheme is based on the attempts of E. Haeckel, F. J. Bell, J. W. Gregory, B. Stürtz, J. O. E. Perrier, and A. E. Verrill. Extinct families marked †.

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.

Grade B. Colophiurae.—Ambulacral pairs fused to form vertebrae with definite articular surfaces; mouth, radial and genital shields developed, though not all need be present in any one form.

Order 1. Streptophiurae.—Rays simple and capable of coiling, since the vertebrae articulate by a ball-and-socket joint; arm-plates incompletely developed. Families: † Onychasteridae, Ophiohelidae, Ophioscolecidae, Ophiomyxidae, Hemieuryalidae, Astrophiuridae; unclassified genera, e.g. Ophioteresis, Ophiosciasma, Ophiogeron.

Order 2. Zygophiurae.—Rays simple and prevented from coiling by processes on the vertebral joints (fig. 18); dorsal, ventral and lateral arm-plates present.

 Sub-order i. Brachyophiurae.—Spines short, simple, pointing towards the end of the arm. Families: Pectinuridae (= Ophiodermatidae), Ophiolepididae.

Sub-order ii. Nectophiurae.—Spines may be variously elaborated and are set more at right angles to the arm-axis. Families: Amphiuridae, Ophiacanthidae, Ophiocomidae, Ophiothrichidae.

Order 3. Cladophiurae (= Euryalae). 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. There are five prominent mouth-shields and a separate madreporite on the ventral surface. These genera attained the Colophiuran grade in respect of external plating, but it is unlikely 