THE METAMORPHOSIS OF ECHINODERMS. 113 
the primary genital pore lay. It also agrees with the direc- 
tion of the growth of the rachis round the disc in ontogeny. 
Whether the Skeleton was one of the first structures to 
assume radial symmetry, or whether this symmetry has been 
independently acquired by Echinozoa and Pelmatozoa respec- 
tively, is a question which is still too much sub judice for me to 
attempt to decide it. It may freely be admitted that when once 
the new symmetry was thoroughly impressed upon the body, any 
set of organs might acquire it in independent groups (we shall 
see directly that this has been the case) ; so that there is no 
reason why homoplasy should not be responsible for all the 
supposed fundamental homologies that P. H. Carpenter and 
others tried to establish. But between this and asserting that 
all the apparent resemblances are homoplastic, there is a very 
great difference. I admit that we cannot rely upon the 
skeleton alone to settle this question; but until we have 
arrived at far more certainty than at present as to the rela- 
tions of the various classes of Echinoderms, I do not think 
we are justified in absolutely denying the possibility of homo- 
logy between, for example, the dorso-central and basal plates 
of Crinoids, and the similarly-named plates in Echinozoa. We 
ought rather to return an open verdict and wait for fresh 
evidence. 
The utter unimportance of this question to my hypothesis 
led me, as far as convenient, to omit all mention of the 
skeletal plates in the first part of this paper; one or two bits 
of fresh evidence, however, there introduced are worthy of con- 
sideration. In the first place there is not the smallest 
embryological ground for Neumayr’s statement (25, p. 498) 
that the primitive arrangement of the plates in Echinids is in 
rows of ten; indeed, the invariable development of all plates 
in Echinoderm larve not directly connected with the ambu- 
lacral system in groups of five, is a very striking coincidence if 
the symmetrical arrangement has arisen out of primitive chaos, 
at various times and in ancestors of various sizes, independently ; 
while it is easily intelligible if the number five was established, 
at any rate in a few plates, at that early stage when the 
vol. 38, PART 1.—NEW SER. H 
