148 ECHINODERMS OF THE BRITISH ISLES 



The tube-feet always lack a sucking disk, but may be covered 

 with sensory papilla? ; they play only an unimportant role, or 

 none at all, as locomotory organs. The first pair of tube-feet are 

 always situated in the depth of the mouth, near the point of the 

 " jaws ", viz. the five projecting mouth parts ; the second pair 

 is situated either near the edge, but still within the mouth, or 

 wholly outside the mouth edge (as in Ojphiura, Fig. 84). At the 

 end of the arm there is an unpaired, terminal feeler, which can 

 protrude through the tube-shaped terminal plate. No eye-spot 

 is found on the terminal feeler. 



The mouth, which is on the under-side, is star-shaped (Fig. 84) ; 

 along the edges of the five projecting parts, the jaws, are usually 

 found some papillae, the mouth-papillce ; on the inner point of 

 the jaw there are some other papillae, either arranged in a single 

 vertical series, or in several more or less regular series ; when 

 arranged in a single series they are designated as teeth, when in 

 several series tooth-papillce {e.g. Ophiothrix, Fig. 98). Not rarely, 

 however, irregularities occur in species with normally a single 

 series of teeth, so that something resembling tooth-papillae results ; 

 this may occur especially in larger specimens. The tooth papillae 

 occupy only the outer part of the jaw ; deeper within the mouth 

 they continue in a series of teeth, which may have a strong cap 

 of enamel. Outside the mouth papillae there is on each side an 

 elongated plate, the adoral plate, and in the angle between these 

 a somewhat larger plate, the mouth shield, the shape of which is 

 of great importance in classification. One of the mouth shields 

 generally is slightly irregular and has one or a few pores near the 

 edge. This is the madreporite. In many cases it is, however, 

 very difficult to recognise as such, the pore being hidden under 

 the edge of the plate. 



Along the inner part of the arms are found in the interradii 

 on the ventral side a pair of narrow sHts, each leading into a 

 large, thin-walled sac, the bursa (Fig. 85), which has a respiratory 

 function. This bursal or genital slit as a rule extends from the 

 mouth shield to the edge of the disk ; often it carries a series of 

 small papillae, the genital papillce, along its interradial edge. A 

 pair of larger, elongated plates, the genital plates, are situated along 

 the edges of the sUt, connected by a joint at the outer end 

 of the slit ; from here they extend farther out to the edge of the 

 disk, where they articulate with the inside of the radial shields. 

 The genital plates are, however, often indistinct. A few Ophiuroids 

 lack bursae and bursal slits completely. Some, mainly tropical, 

 forms (Ophioderma) have two slits along each side of the arm ; 



