188 



BRITISH PALEOZOIC ASTEROZOA. 



As a result of these and similar observations I think that it will be possible to 

 show that the shapes of the ambulacral ossicles are moulded by the stresses set up 

 by muscular action. 



The Mouth-Frame. — I think that it is also clear that the differences in the 

 mouth-frames in the various Asterozoa are expressions of different habits. In 

 order to show this it is necessary to supplement the accounts already given in the 

 Introductory Section of this monograph. Viguier's description of the mouth- 



128 

 P. 



R. 



At. 



129 



R 



H, 



M; 



D. 



M. 



■* 



Text-fig. 128. — Apical view of the central region of the ambulacralia of Slel/aster equestris. (The outer 

 portions of the ossicles are shown by dotted lines — compare Text-fig. 6, p. 13.) Jr., angle at line of contact 

 of two ambulacralia ; D., distal direction ; H., hollow in which is inserted proximal end of a dorsal longi- 

 tudinal muscle ; M., hollow for dorsal transverse muscle ; P., proximal direction ; R., ridge below which 

 is inserted distal end of a dorsal longitudinal muscle, x 12. 



Text-fig. 129. — Aboral profile view of an ambulacral of Stellaster equestris. Lettering as in Text-fig. 121. 



*, wearing surface, x 20. 



frame in the recent Asteroidea is classical (100). The following revision is in 

 accordance with somewhat fuller knowledge. 



We can conveniently divide the description into two parts : (a) The morpho- 

 logical constitution ; (b) the disposition of the soft parts and the functional 

 activities of the structures. 



(a) The Morphological Constitutor. 



The mouth-frame is composed of twenty elements, ten of which are radial (the 

 first ambulacralia) and ten are interradial (the mouth-angle plates). All these 

 are intimately connected to make a firm skeleton. Viguier in his description lays 



