112 HENRY BURY. 
tentacles, free from the rest of the body, they were forced to 
spread over the oral surface, which thus became divided up 
into radial and interradial areas. This spreading of the 
ambulacral area, which to a large extent seems to have 
occurred independently in Echinozoa and Pelmatozoa, has 
apparently been the cause of that “radial segmentation ” of 
various organs which I shall now attempt, very briefly, to 
follow. 
It is probable—though we have no direct evidence on the 
subject—that the concentration of a portion of the nervous 
system into a ring round the mouth took place at an early 
period, and that this ring supplied the tentacles. When the 
water-vascular system spread over the surface of the body, the 
nerve-bands would naturally accompany it, and thus we should 
get a definitely radial arrangement of the nervous system. A 
special development of the muscular system along the same 
lines might be expected, and so it too would exhibit radial 
symmetry. 
The Generative Organs appear to have followed the new 
(radial) symmetry at a fairly early period. What form they 
assumed in the bilateral ancestor we are not in a position to 
assert ; they may very possibly have consisted, as they do now 
in Elasipoda, of a pair of glands opening by a median aperture 
on the dorsal side, just behind the water-pore; then, when 
radial symmetry was established, other pairs of glands (one 
pair for each interradius) may have been developed, connected 
together by the genital rachis ; but whether this segmentation 
of the gonads took place in the common ancestor, and was 
afterwards lost in Holothurians, or whether the latter branched 
off from the parent stock before this was accomplished (and 
therefore before the separation of Echinozoa and Pelmatozoa), 
I am quite unable to determine. In any case the assumption 
of a primary pore just behind the water-pore (or rather, being 
median, behind the dorsal sac), enables us to understand why 
it is, as I have endeavoured to show, that the genital rachis 
marks the original line of division of the body-cavities—its 
growth simply following the line of the mesentery, in which 
