Dr. F. A. Bather — Studies in Edrioasteroidea. 213 



suppose the Edrioasteridae to be descended from forms with a more 

 definite organ of attachment and -with a moderately developed aboral 

 nerve-system, we shall none the less expect to find that system 

 considerably atrophied and leaving but slight traces. If, on the other 

 hand, we suppose that the Edrioasteridae received their aboral 

 nerve-system directly from the supposed anterior nerve-ganglion of 

 some pre-echinodermal ancestor, Dipleurula or other, then again this 

 will scarcely have been modified far in the direction of a motor centre. 

 The same remark applies to any imaginable derivation of the 

 Edrioasteridae from some echinoderm other than a Pelmatozoon, say 

 a primitive Asterozoon. 



The proof that the Edrioasteridae are Pelmatozoa in the full 

 signification of that name has been laboured because the striking 

 characteristic of this Family, more than of any other Edrioasteroidea, 

 is just that strange resemblance to Asteroidea which suggested the 

 second component of the Class name, and which will be fully discussed 

 in the next Study. 



Let us now consider more closely the Eorm and its relation to 

 Fixation. 



Edrioaster, Leletodiscus, and Dinocystis have their rays curved like 

 those of Agelacrinus, Lepidodiscus, and similar Agelacrinidae. This 

 ensures a more or less circular outline of the theca or at all events 

 checks any tendency to a star shape. That this curvature is 

 secondary is an obvious conclusion from a comparison with the 

 Agelacrinidae, from the facts of individual growth (Study IV, 1914, 

 pp. 120, 121), and from the existence of the straight-rayed Stegano- 

 blastus. Further, the oldest known Edrioasteroid, Stromatocystis 

 from the Middle Cambrian, has straight rays. This last form is 

 pentagonal, with a tendency to be stellate; but it cannot therefore 

 be inferred that its as yet unknown ancestor was more stellate. 

 The assumption of a pentagonal or star shape is a consequence of 

 the straight outgrowth of the food-grooves. All that we know of the 

 evolution of those structures in the earlier Pelmatozoa indicates that, 

 in race-history as in individual growth, they stretched gradually 

 outwards from the mouth, and by degrees impressed a radiate 

 symmetry on every part of the theca as well as on the internal organs. 

 Stromatocystis, as will appear in a subsequent Study, affords no 

 evidence of descent from an ancestor in which the radiate symmetry 

 was more strongly marked than in itself. On the contrary, for this 

 Cambrian genus, as for all the Ordovician Edrioasteroids, the natural 

 inference is that they are descended from a simple sack-like, 

 irregularly plated pelmatozoon with no more than the beginnings of 

 radiation seen in its food-grooves, which at first were but three in 

 number. (See Treatise on Zoology, 1900, p. 11; Study I, 1898, 

 p. 545; Study Y, 1914, p. 201; and A. Foerste, 1914, op. cit., 

 p. 412.) 



It is generally admitted that radiate, symmetry of this kind can only 

 have arisen in a fixed form, and that in the case of Pelmatozoa the 

 fixation must have been by the apical end. Such fixation was 

 retained, or perhaps emphasized, in Cyathocystis and Steganollastus. 

 Stromatocystis, however, the only Edrioasteroid as yet known from 



