INTRODUCTION. 37 



sliow tliat Salter and (xroo-ory were inistaken, and that the niadreporite is actinal 

 in Lapiijorfhfird, being in fact idniost exactly in the .same position as is the opening 

 of the stone-canal in Recent Ophinroidea (Sollas and Sollas, 71, p. 21 (i). Further, 

 these two authors have shown that Ch'egory was mistaken in his quotation of 

 Bury, " who was speaking not of the young Ophiuroid but of the larva" (71, 

 p. 215). The observations which caused Stiirtz such misgivings may therefore be 

 ignored. 



My own observations appear to show that Stiirtz was very nearly correct 

 when he assumed that the ventral position of the madreporite was the primitive 

 one, and that the dorsal position found in the Asteroidea has Ijeen assumed 

 secondarily. 





MA.P 



Text-fig. 30. —View of inter- radius of Palseastcrltia priinxva. I. R., inter-radial platos ; M., madreporite ; 

 M. A. P., mouth-angle plates ; Ad., adaml)ulacralia. 



If we consider the three groups, the primitive Asterozoa, the Ophinroidea and 

 the Asteroidea, we find that in the first two of these the madreporite, when known, 

 is almost always in the same position, namely, somewhat excentric to a median 

 line drawn through an inter-radius and adjacent to the first two or three adambu- 

 lacralia (PI. I, figs. 8 and 10). In the Ophinroidea the madreporite (on one of the 

 buccal shields) is on the oral surface and there is a large area of the disc external 

 to it (Text-fig. 12). If this arrangement were the primitive one, the madre- 

 l)orite would have had to travel over a large surface before it reached a secondary 

 dorsal position, and we should expect to find traces of the migration. 



Fortunately, the observations of MacBride help us out of the difficulty. 

 MacBride has shown (41) that the Ophiuroid disc is a secondai'y growth of the 

 aboral parts of the interradii which grow round to form pouches for the reception 

 of the stomach and for space for the genital bm-sas. Primitively, the Ophinroidea 

 had a very small disc. Further, the overgrowth of the disc has led to the forcing 

 of the madreporite from a marginal to an oral position. 



MacBride's views are supported by investigations upon the primitive Asterozoa. 



