Ophiopholis is distinguished from the foregoing genera by the granular covering of its dorsal plates; whilst in Ophiacantha these granules develop into prominent spinelets, and the arm-spines are also thorny. Ophiopholis aculeata occurs in swarms in the branches of the Firth of Clyde, and presents a most remarkable series of variations in colour. Ophiopsila is a closely allied form, distinguished by its large peristomial plates.
Fig. 221.—Oral view of Ophiacantha chelys. × 4. (From Wyville Thomson.)
Fam. 3. Ophiocomidae.—Both mouth-papillae and tooth-papillae are present;[[468]] the arm-spines are smooth, and the disc is covered with granules.
Ophiocoma nigra is the only common British representative of this family. In this species the plates of the dorsal surface are completely hidden from view by a covering of granules. Ophiarachna.
Fam. 4. Ophiothricidae.—Tooth-papillae alone present, mouth-papillae absent; arm-spines roughened or thorny.
This family is represented only by Ophiothrix fragilis, which is perhaps the most abundant of all British Ophiuroids, and has been selected as the type for special description.
The back is covered with spinules, having, however, the triangular radial plates bare. This produces a contrast-effect, which suggested the name pentaphyllum, formerly used by some naturalists for the species. It occurs in swarms, and presents variations in colour nearly as marked as those of Ophiopholis. Ophiopteron is probably a swimming Ophiuroid, as the lateral spines of each segment of the arm are connected by a web of skin.
Order III. Cladophiurae.
