276 E. W. MACBRIDB. 



Therefore I feel that we are shut up to the supposition that 

 Asterids had a fixed ancestor, and we must suppose that this 

 form lived under conditions where enough food drifted along 

 the bottom to meet its demands. PL XXII, fig. 157, represents 

 the characters which I consider the common ancestor of all 

 Echinoderms possessed when it became fixed. Figs. 158 and 

 159 show how these characters became modified in the cases of 

 the Asterid and Crinoid respectively. 



It is probable that a fixed stage occurs in the life history 

 of all Asterids. The larvae of Echinaster and Asterias 

 Miilleri, which are carried in brood-pouches, certainly possess 

 one, and the three papillae on the Brachiolaria larvae are 

 generally interpreted as an apparatus for fixation. 



The fixed stage has, however, been lost so far as we know in 

 all other Echinoderms ; and it is instructive to note in this 

 connection that Asterids alone retain the great prseoral lobe. 

 This has completely atrophied in the Plutei both of Ophiurids 

 and Echinids ; and in the latter case, as I have indicated above, 

 (page 273) there is some evidence to show that a prseoral ciliated 

 band has likewise disappeared. The Auricularia still retains 

 a trace of the prseoral lobe, and it has been regarded as an ex- 

 ceedingly primitive form because it retains the undivided lon- 

 gitudinal ciliated band, and because the larval mouth becomes 

 the adult one. The internal anatomy of this larva shows that, 

 except in these two points, it is the most modified of all ; the 

 anterior coelom so conspicuous in the Bipinnaria is represented, 

 as Bury has shown (2), by a bud of cells which forms the 

 secondary madreporite on the stone-canal, and the whole mode 

 of segmentation of the coelom is most erratic. 



I have dwelt on this subject at some length because some 

 have regarded the Holothurids as the primitive group of the 

 Echinoderms, and Setnon (19) has even attempted to show that 

 the primary hydrocoele lobes in them became the oral tentacles, 

 whilst the so-called radial canals were really interradial out- 

 growths. Ludwig (13) has, however, shown the incorrectness of 

 this ; in the Synaptidse alone do the oral tentacles spring 

 direct from the ring-canal, and it was the development of 



