THE METAMORPHOSIS OF ECHINODERMS. Ez 
It would be altogether beyond the scope of the present paper 
to try and follow further the separation of the different classes 
of Echinoderms. The special features of the Pelmatozoa 
and Holothurians will be briefly considered in tbe following 
pages: but the exact period of separation of these and the 
remaining classes seems to me to require much further evidence. 
I will only add here, as following closely upon the point we 
have just been discussing, a protest against Cuénot’s (7) 
attempt to attach classificatory and phylogenetic value to 
the persistence or closure of the anus in ontogeny ; it may or 
may not possess this value, but in the present state of our 
evidence it would seem almost as reasonable to attach import- 
ance to the persistence of the larval cesophagus in Ophiurids, 
some Asterids, and some Holothurians, or to the entire absence 
of the intestine in the young stages of Antedon and Asterina, 
as opposed to its invariable presence in all pelagic larve. 
Origin of Pelmatozoa. 
I have already given my reasons for believing that the 
Echinozoa were never fixed by the aboral pole, and that the 
utmost that embryological evidence will allow of is a fixation 
of the bilateral ancestor by the przoral lobe; and even this 
fixation, if it had been complete, and not a mere voluntary 
process effected by means of a sucker, would, in my opinion, 
have left more traces behind it than the study of embryology 
has yet afforded us. The existence of such a sucker is certainly 
possible, though the evidence for it is weak, and I do not 
think the assumption of it necessary. But before pursuing 
the subject further it is worth while to point out that whether 
the idea of primitive fixation is accepted or rejected, it remains 
certain that the changes of symmetry which we have been 
considering in the foregoing pages are common both to 
Echinozoa and Pelmatozoa ; and since the acceptance of this 
idea, in whatever form, does not provide us with any explana- 
tion of how these changes came about, my hypothesis—which, 
indeed, only deals with the nature, not the cause, of these 
changes—is obviously independent of it. 
