ZYGOPHIURAE 



499 



corpuscles, carrying haeiiioglobin/ the respiratory value of which 

 compensates for the loss of the genital bursae, which have 

 entirely disappeared. 



Ophiojjholis is distinguished from the foregoing genera by the 

 granular covering of its 

 dorsal plates ; whilst in 

 Ophiacantka 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. 

 Opliiop>sila is a closely allied 

 form, distinguished by its 

 large peristomial plates. 



Fam. 3. Ophiocomidae. 

 — Both mouth-papillae and 

 tooth-papillae are present ; ^ 

 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. 



Fig. 221. — Oral view of Ophiacantka clielys. 

 X 4. (From Wyville Thomson.) 



the arm-spines are smooth, and the 



1 Cuenot, "Etudes Morphologiques sur les Echinodermes," Arch. Biologie, xi. 

 1891, pp. 568 et seq. 



' This type of mouth-frame is represented in Fig. 215, A, by a figure of 

 Ophioscolex, which belongs to the Streptophiurae. 



