184 BRITISH PALEOZOIC ASTEROZOA. 



transverse ventral muscles. Thus far the description is equally applicable to 

 Edrioaster." Bather then goes on to point out that Edrioaster has podial pores, 

 while, in the primitive Asterozoa, I alleged them to be absent. I have already 

 dealt with his statements (see above, p. 67). Sckuchertia, n. sp., quoted there is 

 S. wenloclci of this section. Podial pores are undoubtedly absent in certain 

 primitive Asterozoa, e. g. in Platanaster ordovicus (p. 174), but they are present in 

 more of these early forms than at one time I supposed. It still seems to me, 

 however, that they arose independently in Edrioaster and in the Asterozoa. Bather 

 would, I think, regard the impulse towards endothecal ampulla? as common to the 

 Asterozoa and Edrioaster, and make their existence merely dependent on the 

 existence in the rays of a thecal cavity large enough to contain ampullae (op. cit., 



Ad. 



V.T. 



muse. 



\ B.Tf 



Am.Ch. 



Text-fig. 125. — Floor and inner wall of the ambulacral groove of Archaster fy picas. Ad., adambulacral ; 

 Am.Ch., ambulacral channel; B.Tf., passage for branch water- vascular vessel to tube-foot; D., distal 

 direction ; /., articulation of the adambulacral with its own ambulacra ; it'., articulation of the adam- 

 bulacral with the following ambulacral; P., proximal direction; I'.T.mu.ic, ventral transverse muscle. 

 x 10. This figure may be compared with Text-fig. 5, p. 12. 



p. 318). P. ordovicus has a large thecal cavity but no passages for endothecal 

 ampullae. I might also add that I cannot see that the question of presence or 

 absence of endothecal ampullae is of great importance in studying genetic affinity. 

 Bather also shows (op. cit., pp. 318, 319) a resemblance between the adambula- 

 cralia and the covering-plates. He states that the main point of distinction is that 

 " the adambulacrals of starfish do not close over the groove in the same way as 

 did the cover-plates of Edrioaster, and the contents of the groove are generally 

 protected by groove-spines borne on the adradial margin of the adambulacral 

 plates. None the less, by the approximation of the two sides of the groove, 

 eifected by the ventral cross-muscles, the adambulacrals may be brought quite 

 close together, and in some species they may when thus closed be observed to 

 alternate just like the cover-plates of an Asteroid. See, for example, Ludwig, 



