OPHIUROIDEA 203 



also the radial shields are generally, but not constantly, con- 

 tiguous in their distal part. Not one of these characters is con- 

 stant, and as other, better characters do not seem to exist, the 

 result seems indisputable that 0. corallicola is only a synonym 

 of 0. ahyssicola. 



2. Ophiopus Ljungman. 

 (S}Ti. OpJiiaregma G. 0. Sars.) 



Disk with scales only, no spines or granules. Two to three 

 mouth papillae on each side of jaw. Arms 5. No small plates 

 around dorsal arm plates. Arm spines short, conical, none of 

 them hook-shaped. No genital slits, only a small fold along each 

 side of arm, which may be regarded as a rudimentary bursa. ^ 



Only one species knoT^Ti. 



1. Ophiojms arcticus Ljungman. (Fig. 115.) 

 (Syn. Ophiaregma abyssorum G. O. Sars.) 



Disk scales rather coarse, those of the ventral side not much 

 smaller. Primary plates as a rule fairly distinct. Radial shields 

 very small, separated by a wedge of scales. Dorsal arm plates 

 elongate-triangular, with outer edge rounded ; the proximal ones 

 contiguous, with truncate inner end. Ventral plates with 

 rounded outer edge, farther out wdth a slight concavity in the 

 middle ; the proximal ones contiguous. One rather large 

 tentacle scale. Four, farther out 3, subequal arm spines. Mouth 

 shield rounded triangular. Two large outer mouth papillae, some- 

 times also a very small inner one. Colour in alcohol bluish-grey 

 or whitish. Diameter of disk scarcely surpassing 5 mm. ; arms 

 ca. five times that length. 



The eggs are extruded through small, round holes formed in 

 the ventral side of disk, one to each ovary, not symmetrically 

 situated. The large size of the eggs (ca. 0*4 mm.) indicates direct 

 development, without an Ophiopluteus stage. 



1 Ophiactis virens (M. Sars) also lacks the genital slits or has only a 

 similar fold along the arms as Ophiopus. While the species 0. virens is 

 very easily distinguished from the 5-armed Ophiopus by its 6-7 arms, 

 the lack of genital slits in the said species deprives the main character 

 of the genus Ophiopus of its real value. In fact the only character then 

 remaining to distinguish Ophiopus from Ophiactis is the absence of spines 

 on the disk in the former. It may be questionable whether O. virens 

 can really remain within the genus Ophiactis — there are also some excep- 

 tional features in its internal structure ; but this cannot be decided before 

 the anatomy of the other Ophiactis species has been more carefully 

 investigated. 



