THE METAMORPHOSIS OF ECHINODERMS., LHS 
We are strongly reminded by the above phenomena of the 
varying degrees in which metameric segmentation has been 
acquired in other animals. Indeed Bateson (8, p. 432) regards 
the rays of Echinoderms as a successive, not a truly radial series. 
This is not invariably true in ontogeny; and if the explanation 
given above of the origin of radial symmetry in Echinoderms is 
correct (though I am far from asserting that it is), it will be 
seen that this symmetry was acquired by all organs except the 
water-vascular system in the radial stage, and that therefore 
the existence of pentamerism in the skeleton of bilateral larve 
is precocious. 
We have seen that, in the remote ancestors we have been 
considering, there was probably but little difference in size 
between the two body-cavities ; indeed, if the position of the 
anus may be taken as an indication of the level of the mesen- 
tery, the right body-cavity was, in many of the simpler Cystids, 
much larger than the left, as in fact it is in the “ Cystid”’ stage 
of Antedon. In Echinids and Asterids, however, the growth 
of the ambulacral area (connected, as we have seen, with the 
left cavity only) has led to an enormous preponderance of the 
left over the right cavity. Hence the bulk of the alimentary 
canal came to lie in the region of the left cavity, and it need 
not surprise us if we find the mesentery forsaking its old posi- 
tion and following where it is most needed. This seems to 
have happened in Kchinids, in which the intestinal mesentery 
undulates up and down in the region of the left cavity ; 
while the old line of separation of the cavities is still marked 
by the genital rachis, and, to a less extent, by the skeletal 
plates. 
In Asterids the mesentery probably retained its original 
position for a longer period, the stomach being supported by 
the septa; but with the outgrowth of the hepatic ceca into 
the arms, it too forsook its former position, which the genital 
rachis now alone marks. 
The curious distribution of the two cavities in Ophiurids (as 
indicated by the genital rachis) may perhaps be due to an early 
