92 HENRY BURY. 
But if, on the other hand,as MacBride’s figure (fig. 2) indicates, 
only the left hydroccel of the bilateral ancestor had tentacles, 
then these tentacles, running down the left side, without any 
special relation to the mouth, occupied an unparalleled and, to 
my mind, most improbable position. Moreover, the position 
assigned to them in this figure, though suiting fairly well the 
larva of Asterina, is not at all in accordance with what we find 
in Ophiurid Plutei, in which the hydroccl surrounds the ceso- 
phagus at right angles to the dorsal mesentery, before 
bilateral symmetry is lost. I shall endeavour to show later that 
Ophiurids are more likely to be primitive in this respect than 
Asterina. 
(2) MacBride’s idea of the fixation of the ancestor, followed 
by different changes in Echinozoa and Pelmatozoa respectively 
(19, p. 434), will not suit paleontologists, but is for all that 
more likely, in my opinion, to be right than those theories (such 
as Semon’s, Biitschli’s, &c.) designed to satisfy the supposed 
teachings of paleontology, to which we shall return. It must, 
however, be borne in mind that it rests solely on the fact that 
Asterina, as well as Antedon, becomes fixed by the preoral 
lobe; no trace of such fixation has yet been found in any other 
Echinozoan larva. 
(3) We have seen that in post-larval stages of all Echino- 
derms, except Holothurians, the mesentery of the water-tube 
(formed, as MacBride rightly recognises, by the left body-cavity 
only) is at right angles to the original longitudinal mesen- 
tery of the bilateral stage, while the water-pore is almost at 
the level of this longitudinal (now transverse) mesentery. I 
am at a loss to understand how this condition can be brought 
about by any amount of “ predominance of the left side.” No 
doubt the greater this predominance the more nearly will this 
condition be approached, but it can never beactually reached, and 
the hypertrophy must be enormous before anything like it is 
attained. But what is the evidence of this hypertrophy ? 
No doubt it occurs early in Brachiolaria and Asterina, and 
perhaps too in Echinid Plutei; but, on the other hand, metamor- 
phosis seems to be accomplished in Ophiurids without any 
