One genus, Ophiura, is fairly common round the British coast, being represented by O. ciliaris and O. albida; the former is the commoner. An allied species dredged by H.M.S. "Challenger" is represented in Figs. 217 and 218.
Ophiomusium (Fig. 219) is a very peculiar genus. The mouth-papillae on each side of each mouth-angle are confluent, forming a razor-like projection on each side of each mouth-angle (Fig. 220). The arms are short, and the podia are only developed at the bases of the arms. Ophiopyrgus has the dorsal surface raised into a conical elevation protected by a central plate surrounded by five large plates.
Fig. 219.—Aboral view of Ophiomusium pulchellum. × 7. (From Wyville Thomson.)
In the remaining four families the arms are inserted on the under surface of the disc; in other words, the interradial lobes which make up the disc have completely coalesced dorsally; and the spines stand out at right angles to the arm.
Fam. 2. Amphiuridae.—Mouth-papillae present, but no tooth-papillae; radial shields naked; small scuta buccalia.
The most interesting Brittle Star belonging to this family is Amphiura squamata (elegans), a small form, with a disc about ¼ inch in diameter covered with naked plates. It is hermaphrodite and viviparous, the young completing their development inside the bursae of the mother. Occasionally the whole disc, with the exception of the mouth-frame, is thrown off and regenerated. This appears to be a device to enable the young to escape. Three other species of Amphiura are found in British waters.
Fig. 220.—Oral view of Ophiomusium pulchellum. × 7. (From Wyville Thomson.)
Ophiactis is another genus belonging to this family, distinguished from Amphiura by its shorter arms and smoother arm-spines. It lives in the interstices of hard gravel. The British species, O. balli, presents no special features of interest, but the Neapolitan O. virens is an extraordinary form. It has six arms, three of which are usually larger than the other three, for it is always undergoing a process of transverse division, each half regenerating the missing part. It has from 1 to 5 stone-canals, the number increasing with age; numerous long-stalked Polian-vesicles in each interradius, and in addition a number of long tubular canals which spring from the ring-canal, and entwine themselves amongst the viscera.[[466]] All the canals of the water-vascular system, except the stone-canals, contain non-nucleated corpuscles, carrying haemoglobin,[[467]] the respiratory value of which compensates for the loss of the genital bursae, which have entirely disappeared.