146 ECHINODERMS OF THE BRITISH ISLES 



branched arms. The number of arms is almost constantly 5, 

 though 4- and 6-armed specimens may occur as abnormalities. 

 Only very few species have normally 6 or more arms. The arms 

 are provided with an internal skeleton, very much resembling a 

 vertebral column, made up of originally paired pieces, the 

 ambulacral plates, which coalesce to a single vertebra-like piece 

 provided with articular surfaces and processes for the attachment 

 of the muscles. The articular surfaces are of two different types. 

 In the Euryalids they are hour-glass-shaped, vertical on the proxi- 

 mal, horizontal on the distal end of the vertebra ; this articulation 

 makes possible a vertical movement and the rolling in of the 

 arms. In the other Oj^hiurids it is a more complicated system of 

 knobs and grooves, fitting into one another and in the main 

 allowing only horizontal movements of the arms. A few forms, 

 however, of this group (like Ophiopholis and Ojjhiothrix, as also 

 some Ophiacanthids) are also able to roll in their arms against 

 the ventral side. 



Both disk and arms are, as a rule, covered by a dermal 

 skeleton, consisting of fairly regular plates, the shape and 

 arrangement of which are of eminent importance for classification 

 (Fig. 84). On the dorsal side of the disk the more important are 

 a pair of larger plates off the base of each arm, the radial shields ; 

 very often there are some larger, regularly arranged plates in the 

 middle of the disk, one central plate and around this one or two 

 alternating circles, each with five plates. These primary plates 

 are nearly always to be observed in young specimens, but in most 

 species they become indistinct with age. In some forms the 

 dorsal side is covered by granules or small sj)ines or by a thick, 

 naked skin. On the under- side of the disk the interradial areas 

 generally have the same sort of covering as the dorsal side. A 

 single or double comb of papiUse may occur at the base of the 

 arms. 



The arms are mostly covered by regularly arranged plates, 

 four to each joint or segment, viz. one dorsal, two lateral (or side) 

 plates, and one ventral plate. The ventral and dorsal plates form 

 a regular longitudinal series along the ventral and dorsal mid-line 

 of the arms. The first ventral j^late and the proximal dorsal 

 plates differ somewhat in shaj^e from the following, and towards 

 the end of the arm the plates again gradually assume another 

 shape. In the descriptions of these plates under the various 

 species the description always refers, unless specially stated, to the 

 plates of the inner part of the arms, where they have their normal, 

 not modified, shape. Some Ophiuroids have the arms covered 



