5*7 



CLASS OPHIUROIDEA. 



Echinozoa with a disc-like body and slender arms sharply marked off 

 from it ; arms plated; no ambulacral groove ; madreporite on actinal sur- 

 face, ttsually fused to an oral plate ; tube-feet pointed, lateral in position ; 

 no anus ; genital bursae at base and sides of arms, interradial. 



The character of the integument, the number and arrangement of the 

 plates on the body are subject to much variation. The extent to which 

 the apical system retains the typical arrangement, or is obliterated by 

 subsequently formed plates, differs much even within the limits of the 

 same genus. When it is traceable, there is a dorso-central plate sur- 

 rounded either (i) by a ring of five radials which typically appear before 

 the remaining apical plates ; or (2) by a ring of five basals (Ophiomitra 

 exigud) ; or (3) by basals and radials with or without (4) superadded under- 

 basals. There is often a circle of five primary interradials (inter-brachials), 

 each plate touching a basal and thus separating the radials from one 

 another. Second, third, &c. interradials carry on the line and abut on 

 the actinal surface against the five large orals known as buccal shields. 

 These shields are as a rule very distinct, and are absent only in the 

 sub-class Euryalida with the exception of Trichaster. The integument 

 of the disc is soft in the Ophiomyxidae. Calcareous granules and spines 

 may cover the outer surface of the plates. In the Euryalida plates are 

 absent, and granulations cover the surface and frequently bear attached 

 spines. It is the only group of Ophiuroidea in which structures resembling 

 pedicellariae have been found. They occur singly on the ventral side of 

 certain pores for the tube-feet, and take the form of a pedicle bearing 

 two hook-like valves which are however not apposed but placed side by 

 side. The arms are distinct at their base ; they are branched in the 

 Astrophytidae, and in this family as well as in other Euryalida their 

 extremities, owing to the absence of calcareous shields, can be rolled up 

 towards the mouth. They are covered in the Ophiurida by a single 

 series of dorsal shields, of right and left lateral or adambulacral shields, and 

 of ventral, super -ambulacral or ambulacral shields. A pair of large shields, 

 termed * radials/ but not to be confused with the apical radials, overlie the 

 bases of the arms dorsally. The adambulacral shields are large, their 

 distal margin projecting and generally provided with spines. An aperture 

 between the edge of one shield and of its successor, and the corresponding 

 super-ambulacral gives exit to the tube-feet. It is often surrounded by 

 a ring of small plates. The lateral shields appear about the same time 

 as the vertebral ossicle : and, in Amphiura, the ventral a little before 

 the dorsal shield. The terminal plate becomes tubular and grows round 

 the tentacle (azygos tube-foot) at the tip of the arm. Within the arms 

 is a linear series of large more or less disc-like vertebral, ambrilacral, or 

 sub-ambulacral ossicles, placed dorsally to the radial water-vascular trunk. 



