THE METAMORPHOSIS OF ECHINODERMS. 93 
marked change in the proportions of the two body-cavities ; 
while in Crinoids it actually ends in the left cavity being 
smaller than the right. On Holothurians I offer no opinion, 
but I fail to see in them any support of MacBride’s views ; 
while in Bipinnaria asterigera, lastly, in which the adult 
relation of hydroceel and body-cavities is assumed long before 
metamorphosis, I have shown that the preponderance of the left 
cavity only arises when the arms are formed, and apparently asa 
consequence of this formation—certainly not as a consequence 
of the growth of the hydroccel and left body-cavity round 
the cesophagus. Here, again, as it seems to me, MacBride 
relies too much on the larva immediately under his notice. 
Those who have approached the question of the origin of 
Echinoderms from a_ paleontological standpoint, have almost 
without exception derived all existing forms from the Cystidea. 
Various genera are pressed into service as ancestral, but at some 
period or other all the Kchinozoa are supposed to have passed 
through a stage in which they are fixed by the aboral pole. 
Of this there is not the slightest embryological evidence, for 
even if we follow MacBride in regarding the fixation of 
Asterina as an ancestral feature, that fixation is by the oral, 
and not by the aboral surface, so that it does not in the least 
satisfy the requirements of the palzontologists. Nevertheless, 
almost all embryologists, apparently out of deference to paleeon- 
tological conclusions, have thought it necessary to assume that 
ontogeny is misleading, and that a period of fixation really did 
take place, of which all traces have since disappeared. 
Now this involves us in a question of fundamental import- 
ance. If paleontologists have really proved beyond any 
reasonable doubt that the Echinozoa are derived from fixed 
ancestors, then ontogeny is misleading ; but if it is misleading 
to such an extent as to obliterate all traces of a process of such 
immense importance, I for my part do not see how we can 
trust it in other particulars, and those who rely upon it for 
indications of phylogenetic history had better re-consider their 
position. (The fixation of Brachiolaria and Asterina obviously 
