30 BRITISH PALEOZOIC ASTEROZOA. 



The Mouth-frame in the Primitive Asterozoa. 



The mouth-frame in some of the most primitive forms is almost diagram- 

 matically simple in character. We may take as an example Eoactis simplex, nov. 

 gen., nov. sp. The specimen of this form figured (PI. I, fig. 4) has been 

 compressed in such a manner that the ambulacral groove has been well exposed. 

 This affords us a good opportunity to obtain a clear view of all the ossicles. It is 

 seen that the ambulacralia and adambulacralia are almost exactly of the primitive 

 shape noticed in Uranaster. There are, however, small articulating pegs which 

 show that there is some slight advance towards the Ophiuroidea. The ossicles of 

 the mouth-frame show scarcely any differentiation. The mouth-angle plates are 

 not conspicuously developed. Owing to the compression of the specimen, they lie 

 somewhat on their side, and it is easy to see that their inner faces are excavate. 

 These excavations, together with proximal excavations of the first ambulacralia, 

 form the cups for the first tube-feet. Probably small oral spines were present. 

 These were, however, so slight that no trace of them is visible. The first 

 ambulacralia are of approximately the same size as their neighboiirs. 



The mouth-parts of forms at any rate allied to the primitive Asterozoa differed 

 very little from those described above. Plate I, fig. 5 shows the adambulacralia 

 and month-angle plates of rrotojxilge.aster caractaci looked at from above. The 

 am])ulacralia have fallen away from this portion of the specimen. 



Hudson's description of other portions of the specimen leaves no doubt that 

 there was neither an ambulacral corresponding to the mouth-angle plates nor an 

 anterior process from the first ambulacral. The position of the cup for the first 

 tube-foot, which can readily l^e deduced from a comparison of the two figures, 

 suggests that the feet themselves were directed slightly forward. They must 

 have been able to thrust themselves freely into the mouth-cavity, and probably 

 helped to pass small particles of food which had been seized by the podia into the 

 stomach. If this were the case the method of feeding could not have materially 

 differed from that of many Pelmatozoa. 



The Advance towards the Asteroidea. 



An advance towards the structures as seen in Recent Asteroidea is shown by 

 the Devonian form Xenaster margaritatns, Simonov., j)ars, figured and described b}'' 

 Schondorf (62). The first ambulacralia are considerably thicker than the 

 remaining ambulacralia and there is a distinct anterior process. 



The 'Report of the New York State Museum' for 1911, p. 44, contains an 

 interesting account of the discovery of a Starfish bed which suggests that in 

 Devonian times the Asteroidea had the same method of feeding as they have at the 



