1915 393 F. A. Bather — Studies in Edrioasteroidea. 



IX. The Genetic Relations to other Echinoderms. 



[Geol. MAG., N.S., Dec. VI, Vol. II, pp. 393-403 ; Sept., 1915.] 



TPHE last Study brought out many resemblances between 

 J_ Edrioasteroidea and Asteroidea, much closer than have been 

 recognized even by some who have suggested a derivation of 

 the latter Class from the former. In the present Study it is 

 proposed to inquire whether that hypothesis, or any similar 

 hypothesis, is tenable. 



Apart from all other considerations, it is clear that, if Edr toaster 

 has in its subvective skeleton reached a stage of development higher 

 than that of the older Palaeozoic Asterozoa, then it cannot be 

 regarded as the ancestor of these latter. Further, it is hardly 

 conceivable that the earliest Asterozoa known to us were derived 

 from a genus entirely contemporaneous with them. But the Middle 

 Ordovician Edrioasteroidea had, we know, ancestors in the Cambrian, 

 so that the group was probably in existence in Lower Cambrian times, 

 if not before ; and it is among those early forms that the ancestors 

 of the Asterozoa must be sought by any who would support the 

 hypothesis under examination. The descendants of those ancient 

 Pelmatozoa exhibited considerable diversity in regard to locomotion, 

 and we do actually find in the Edrioasteridse forms that have 

 assumed many Asteroid characters. Surely there may have been 

 other descendants with a tendency to similar structures, but adopting 

 a mode of life in which those structures found larger scope for 

 exercise and development. We know, it is true, that similarity 

 of form is in itself insufficient evidence of blood-relationship ; and 

 yet there may be something in Mr. Bergson's argument that the 

 independent appearance of similar structures in different groups 

 points to some identity of initial impulse impressed on a common 

 ancestry. 



Time-relations, then, admit the possible derivation of Asterozoa from 

 Edrioasteroidea. But of the three main changes involved in such 

 derivation we have as yet only considered two, namely, the change of 

 function in the ambulacra from nutrient to locomotor, and the 

 elaboration of the mouth from a passive funnel to an active predatory 

 organ. These two changes involve no difficulty, but the third, on 

 which they both depend, is less easy to explain. It is the reversal of 

 the main axis of the body with reference to the substratum ; in other 

 words, the transference of the oral pole to the under surface. 



In a Pelmatozoon the main axis is vertical, the oral pole is 

 uppermost, and around it are the water-ring (from which proceed the 

 perradial canals), the hydropore, the genital aperture (when present), 

 and the anus ; the coil of the gut, when viewed from the oral surface, 

 is dextral or solar. In a primitive Holothurian the main axis is 

 horizontal, the oral pole is anterior, and around it are all the above- 

 mentioned organs except the anus, which is posterior; the coil of the 

 gut is the same. In a regular Echinoid the main axis is vertical, 

 the oral pole is lowermost, and around it is the water-ring, from 



