1. Phanerozonia.—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. Cryptozonia.—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. Ophiurida.—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.

Fig. 17.—A Cryptozonate Asterid, Solaster papposus, from the upper or dorsal surface.

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.

Fig. 18.—A vertebral arm-ossicle (fused ambulacrals) of a Zygophiuran, Ophiolepis.
A, Proximal joint-face. B, Distal joint-face. c, Ventral groove, where lies the water-vessel, from which branches pass through the ossicle, emerging as podia at e and e.