Br. F. A. Bather — Studies in Edrioasteroidea. 317 



In general shape the Edrioasteroidea are more rounded and less 

 stellate than the starfish. This is a natural consequence of the mode 

 of life of the two classes. The Edrioasteroid that appears to have 

 been the least sedentary, namely Stromatocystis, is also the most 

 stellate. Similarly in Asteroidea the active Asteriidae have, as a rule, 

 longer arms than the more sluggish Asterinidse and Echinasteridae. 

 Mere outline, then, has little genetic significance ; the distinction, if 

 any, lies in the large development of interambulacral plates in the 

 Edrioasteroidea and their lack of marginalia, for the ' marginals ' of 

 Agelacrinus are scarcely homologous with those of Asteroidea. The 

 observations of Mr. Spencer, however, " show that the most primitive 

 Asterozoa have a leathery skin in which are imbedded small irregular 

 plates" (p. 34), and that many of them "are devoid of differentiated 

 marginalia" (p. 20). The specialization of the irregular plates into 

 marginalia or other skeletal elements differed in the different branches 

 of Asterozoa (Spencer, p. 8), just as it differed in the various genera 

 of Edrioasteroidea. The ossicles of fundamental importance are those 

 which were " the first to be laid down " or, at any rate, to be 

 specialized from the indifferent coating of plates, namely, "those 

 associated with the water-vascular system (ossicles of the ambulacral 

 groove and mouth-frame)." It is to these, then, that our attention 

 must be directed. 



In the older Asterozoa, from which the true Asteroidea and 

 Ophiuroidea were derived, the plates which in a modern starfish are 

 known as " Ambulacralia " were "little more than mere flooring 

 plates to the ambulacral grove" (Spencer, p. 21). They formed 

 a double series, either opposed as in recent Asteroidea, or alternating 

 as in Edrioaster. Spencer, following Gregory and Jaekel, regards 

 the latter arrangement as the more primitive. These ambulacrals 

 were approximately rectangular in plan, and excavated along the 

 perradial sutures by a shallow "ambulacral channel" for the radial 

 water-vessel. Along the sutures at right-angles to this the plates 

 were deeply excavate, leaving a well-marked median transverse ridge 

 along each. The longitudinal ridge, parallel to the ambulacral 

 channel, was but slight, indicating the feeble development of the 

 transverse ventral muscles. 



Thus far the description is equally applicable to Edrioaster, but 

 the difference from both Edrioaster and the true Asteroidea lies in 

 the alleged absence of podial pores. It is presumed that branches 

 led from the perradial water-vessel to podia placed somewhere in 

 the depressions between the ridges ; and it is possible that these 

 depressions indicate the presence of incipient ampullae; but, according 

 to Mr. Spencer, the ampullae had not yet penetrated to the interior. 

 Were that view correct, Edrioaster itself would be more advanced in 

 this respect than the older Palaeozoic Asterozoa, and would find its 

 analogue in such a form as the Lower Devonian Xenaster, where the 

 pores have the same relative size and position (see Schondorf, 1909, 

 Palaeontographica, vol. 56, pi. xi, fig. 2). 



In reference to the earlier Asterozoa, Mr. Spencer says (p. 18): 

 "Many investigators, owing to the poor state of preservation of 

 their material, have described these depressions as pores for the 



