EOACTINIDiE. 



193 



dorso-lateral pits are raised on to the surfaces of processes, so that there are in 

 all four processes, two of which articulate with one half of the jaw " (MacBride, 

 op. cit., p. 484). These movements are brought about by the intervertebral 

 muscles connecting the basal parts of the jaws (the part behind io.v.v. of PI. I, 



.T.ur.V. 



Text-fio. 134. — Longitudinal section through the disc of an Ophiuroid passing along one arm and the 

 middle of the opposite interradius (slightly modified from Ludwig). -4i- 5 , ambulacrals ; Coe., crelom ; 

 D.musc,, " binding " interdental muscle ; D.musc.*, " thrusting " interdental muscle; P. } , first tube-foot; 

 F. t , second tube-foot; M.A.P., mouth-angle plate; M.S., mouth-shield; n.g., groove for nerve-ring; 

 /•.«., radial nerve; r.m.v., radial water-vessel; T., torus; w.v.r., water-vascular ring; *, approximate 

 position of madreporic pore. 



fig. 2) with the succeeding vertebrae. We can connect these movements with 

 the absence of the odontophor in the Ophiuroidea, for it is obvious that any 

 plate connected with the proximal vertebras and the floor of the disc would 

 impede the free up-and-down movement of the jaws. Another series of 

 movements of the jaws of the Ophiuroidea is brought about by the simultaneous 



D.ITUISC.1. 



D musc.2. 



w. v.r. 



' Pr. MP 



Text-fig. 135. — Wash drawing of interradial surface of mouth angle plate (that turned towards its 

 opposing pair) of Solaster pupposus. Ap., apophysis ; D.musc. u " binding " interdental muscle ; D.musc.%, 

 " thrusting " interdental muscle ; In.M., interradial muscle (muscle " adducteur des dents ") ; n.r., groove 

 for nerve-ring ; Pr. M. P., ascending limb of mouth-angle plate ; iv.v.r., groove for water- vascular ring, x 6. 



contraction of five muscles, each of which unites the two halves of a jaw 

 (MacBride, op. cit.). By this means the mouth can be narrowed and the jaws 

 forced inwards. PL I, fig. 2, shows that the two forwardly projecting portions 

 of the jaws of the same interradius are not adjoining, as are the mouth-angle 

 plates of the Asteroidea, but form two sides of an acute-angled triangle. The 

 muscles in question form the base of this triangle and are the external interradial 



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