THE METAMORPHOSIS OF ECHINODERMS, 121 
similarities between the two groups would justify us in recog- 
nising. With this exception the supposed migration of the 
disc of fixation to the aboral pole would be for the most part 
superficial in its effects, and the general arrangement of the 
internal organs would not be altered by it. 
If a dorso-central plate existed at the aboral pole of the 
ancestor, I see no difficulty in supposing that the disc of fixa- 
tion might come to lie external to it, the animal up to this 
stage remaining sessile; and that when the stalk was formed 
this plate was borne out on the end of it. But whether we 
accept this skeletal homology or not, I do not see sufficient 
erounds for adopting MacBride’s conclusion (19, p. 486) “ that 
the abactinal poles of Asterina and Comatula are not 
comparable with each other, and that all conclusions 
based on the supposed homology of the dorso-central of 
Echinids and Asterids, and that in Crinoids, are incorrect.” 
The exact stage atwhich this fixation may be supposed to have 
occurred is not, perhaps, a matter of great consequence. The 
frequent occurrence in Cystids (some of which may almost 
certainly be regarded as the earliest Pelmatozoa) of twenty- 
five tentacles (five to each radius) suggests that this may have 
been the number reached by the common ancestor of all the 
Echinoderms ; but, as already mentioned, the absence of any 
pause at this stage in most Echinozoa, as well as the apparently 
anomalous order of development uf these twenty-five in Ante- 
don, render this extremely doubtful, and make it perhaps 
more probable that the separation of these two main divisions 
of the Echinodermata took place at a still earlier period, though 
whether at a stage with fifteen tentacles or with only five there 
is no evidence to prove. 
It remains for me to show that the hypothesis put forward 
above is not inconsistent with the apparent teaching of embry- 
ology that fixation took place by the preoral lobe. It seems 
to me that we are apt to speak of this lobe as if it were a defi- 
nite organ, instead of a region of the body possessing a 
great number of parts; and that though Brachiolaria and the 
larva of Antedon are both fixed, apparently, by the przoral 
