This is not a very well defined order; it includes a few genera intermediate in character between the Cladophiurae and the Zygophiurae, and believed to be the most primitive Ophiuroids living. It is not divided into families. The vertebrae have rudimentary articulating surfaces, there being two low bosses and corresponding hollows on each side, and so they are capable of being moved in a vertical plane, as in the Cladophiurae; the arms never branch, and further, they always bear arm-spines and lateral arm-plates at least. No species of this order are found on the British coast, but Ophiomyxa pentagona, in which the dorsal part of the disc is represented only by soft skin, is common in the Mediterranean.
Ophioteresis is devoid of ventral plates on the arms, and appears to possess an open ambulacral groove, though this point has not been tested in sections. Ophiohelus and Ophiogeron have vertebrae in which traces of the double origin persist (see p. [491]).
Order II. Zygophiurae.
This group includes all the common and better-known British forms. They are divided into five families, all of which are represented in British waters.
Fig. 217.—Aboral view of Ophioglypha (Ophiura) bullata. × 3. (From Wyville Thomson.)
Fam. 1. Ophiolepididae.[[464]]—Arm inserted in a definite cleft in the disc, or (expressing the same fact in another way) the interradial lobes out of which the disc is composed are not completely united. Radial shields and dorsal plates naked. Arm-spines smooth and inserted on the posterior border of the lateral arm-plates.
Fig. 218.—Oral view of Ophioglypha (Ophiura) bullata. × 5. (From Wyville Thomson.)
This family includes all the Brittle Stars of smooth porcelanous aspect and provided with only short spines. Forbes[[465]] called them Sand-stars, since their short spines render these animals incapable of burrowing or of climbing well, and hence they appear to move comparatively rapidly over firm ground, sand, gravel, or muddy sand, and they are active enough to be able to capture small worms and Crustacea. The prey is seized by coiling one of the arms around it.