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



In all the early Pelmatozoa, on the other hand, the hydropore, the 

 anus, and presumably the closure of the hydrocoel are normally in 

 a single interradius, which bears a definite relation to the branching of 

 the rays, and therefore serves as the starting-point for their numbering. 



Now since it is not immediately obvious how either of these sets of 

 organs can have jumped over the intervening radius (I), this seems 

 rather a fundamental difference. It is, I believe, capable of 

 explanation on the Pelmatozoic theory ; but before reverting to that 

 let us consider some further facts of embryology. 



First, note that in the recent adult starfish both anus and 

 madreporite are on the apical surface. In the early stages, however, 

 they are on the oral surface. Therefore they have migrated. Now 

 while the madreporite remains between the same rays of the star as 

 the original hydropore lay, the anus has changed. We are, however, 

 unable to trace its migration because there is a gap due to closing 

 of the larval intestine and the formation of a new rectum and 

 proctodaeum. This fact alone should be enough to suggest that 

 the evolution of the Asteroidea from the fixed Dipleurula cannot have 

 been so simple and direct as claimed by Gemmill and MacBride and 

 as represented in the batter's diagrams. 



As regards the madreporite, the application of these facts to 

 phylogeny is clear, because in the fossils we can trace the historical 

 passage of the madreporite between the rays from an adoral, or 

 oro-marginal, position to an adapical position (see Spencer, 1914, 

 Mon. Brit. Pal. Ast.). 



The anus is so obscure, if not absent, that its passage is less easily 

 traced in those fossils. According to C. Schuchert, 1 " The only 

 Paleozoic form in which an anal opening may exist visually is 

 [the Ordovician] JTudsonaster. Here it is on the [adapical] disk 

 between the central plate and the madreporite " (p. 13), apparently, 

 then, still in the hydropore interradius ; but the anal nature of the 

 appearance is very doubtful (p. 39). In other fossil Echinoderms, 

 however, the migration of the anus can be traced and is observed to 

 follow the presumed line of the coiled gut. That is just what 

 happens in the developing starfish : the coil of the gut shortens and 

 the anus is consequently pulled in a contrasolar direction (as viewed 

 from the oral pole), i.e. from the madreporite plane into the Asterid 

 plane. Though the coil of the gut is thus obscured in Asteroidea, it 

 is important to recognize that it really is of the same nature as in 

 other Echinoderms (see Treatise on Zoology, 1900, p. 34). 



Consequently, from the embryological and anatomical facts now 

 placed in so clear a light by Dr. Gemmill, we can readily imagine 

 how and why the anus, in its phylogenetic passage from the oral to 

 the aboral surface, followed a course which was not straight but 

 curved so as to bring it into the interradius where it now lies. 

 Further we see that, whereas the anus was exposed to two forces 

 or tendencies, of which its present position is the resultant, the 

 madreporite was subject to only one, namely the tendency to pass 



1 " Kevision of Paleozoic Stelleroidea," U.S. National Mus., Bull. 88, 

 20 March, 1915. This valuable work appeared some months after these 

 paragraphs were first written. 



