Order 1. Aspidochirota.—Tentacles more or less peltate; calcareous ring when present simple and radially symmetrical; no retractors; stone-canal often opens to exterior; genital tubes sometimes restricted to left side in consequence of altered position of gut (Fig. 15.) Families: Elpidiidae (deep-sea forms, with sub-famm. Synallactinae, Deimatinae, Elpidiinae, Psychropotinae), Holothuriidae (shallow water), Pelagothuriidae (pelagic).

Order 2. Dendrochirota.—Tentacles simple or branched, never peltate; calcareous ring well developed, often bilaterally symmetrical; retractor muscles usually present; stone-canal opens internally; genital tubes in right and left tufts.

Sub-order i. Apoda.—No tube-feet or papillae, but tentacular ampullae more or less developed. Mostly burrowers. Families: Synaptidae (sub-famm. Synaptinae, Chirodotinae, Myriotrochinae), Molpadiidae.

Sub-order ii. Eupoda.—Tube-feet present, but tentacular ampullae rudimentary or absent. Families: Cucumariidae (climbers and crawlers), Rhopalodinidae (burrowers).

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; 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 [Starfish]) 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. Asterida.—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 †.

Fig. 16.—Section across the arm-skeleton of a Phanerozonate Asterid, Astropecten.
a, Ambulacral plates. b, Adambulacral plates. c and d, Inferior and superior lateral plates. e, Dorsal plates with paxillae. Certain supra-ambulacral plates, which also exist, are not shown.