EOACTINID^E. 



191 



published the figure reproduced here as Text-fig. 1.32, stated that he had been 

 mistaken in his opposition to Viguier, and that he would write further upon the 

 point (101, p. 469). The figure shows clearly that there is no first ambulacral 

 proximal to the first ampullal pore. Unfortunately even now the morphological 

 constitution of the mouth-frame of all fossil Asterozoa is not certain. Urasterella 

 and its allies (Text-fig. 87, p. 134) have a mouth-angle plate which may well be a 

 modified first adambulacral. Further research is necessary before all points can be 

 cleared up. The position is, however, quite clear in 



the forms leading to the 



M. 



Text-fig. 133. — A doral view of month-frame of Stellasier equestris. Am!., first ambulacral ; Ap., apophysis; 

 F.,, passage for ampulla of first tube-foot ; M., hollow for dorsal transverse muscle; M.P., mouth-angle 

 plate ; O., odontophor ; Sp., mouth-spine ; n.r. and w.v.r., as in Text-fig. 131. 



Ophiuroidea. Here the mouth-frame has essentially the same structure as that 

 described by Viguier. 



(b) The Disposition of the Soft Parts and the Functional Activities of the Structures. 



We can follow this evolution the more clearly if we learn to recognise the 

 various grooves and excavations which show the disposition of the soft parts. 

 The mouth-frame always carries the water-vascular ring-canal and the nerve-ring. 

 The first-named is carried in a groove above the apophysis partially situate 

 upon the ascending limb and partially upon the inner (adradial) side of the 

 thickened portion of the first ambulacral. The groove for the nerve-ring, as 

 Dr. Gemmill has pointed out to me, is that below (on the adoral side of) the same 

 apophysis. It follows from this that the area between the two grooves is one 

 which we can mark down whether the apophysis is prominent or not, and we shall 

 see that the presence or absence of prominent apophyses will throw considerable 



