394 E. W. MACBRIDE. 
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. PI. 29, 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 larve of Echinaster and Asterias 
Miilleri, which are carried in brood-pouches, certainly possess 
one, and the three papille on the Brachiolaria larve 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 preoral lobe. 
This has completely atrophied in the Plutei both of Ophiurids 
and Echinids ; and in the latter case, as I have indicated above, 
(page 391) there is some evidence to show that a preoral ciliated 
band has likewise disappeared. The Auricularia still retains 
a trace of the preoral 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 ccelom 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 Sémon (19) has even attempted to show that 
the primary hydroceele 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 Synaptidz alone do the oral tentacles spring 
direct from the ring-canal, and it was the development of 
