THE METAMORPHOSIS OF ECHINODERMS. hit 
organs. It was probably increase of size which led to the 
further development of the ambulacral system, and this in its 
turn led to the completion of that radial symmetry which is 
among the most striking possessions of the Echinodermata. 
The first addition to the ambulacral system probably took 
the form of the production of five pairs of tentacles at the bases 
of the original five, so that the total was raised to fifteen. The 
next stage is not so easy to follow. In Crinoids five more pairs 
are added between the previous pairs and the mouth, that is to 
say, in centripetal order; and then, with a total of twenty- 
five, there is a long pause. Many Cystidea never get beyond 
this total, so that we may regard it as primitive, at any rate 
for the Pelmatozoa. 
As regards the Echinozoa, there is something of a pause 
when the number twenty-five is reached in Ophiurids, but not, 
I think, in other groups; and even here there is no evidence to 
show in what order the pairs arise. In all later-formed tube- 
feet the succession in Echinozoa is invariably centrifugal 
(acropetal), and this seems to be the case even from the first in 
Holothurians. In Crinoids, however, a totally distinct order 
is observable. We are then driven to the conclusion that the 
Pelmatozoa branched off at least as early as the stage with 
twenty-five tentacles, and there is some evidence that the 
common ancestor did not get beyond a total of fifteen. 
At this point we may leave the Pelmatozoa, the origin of 
which will be discussed later, and for the present direct our 
attention chiefly to the Echinozoa, in which, so far as we can 
see, the increase in number of tube-feet (tentacles) went on 
pretty steadily, in acropetal order. 
It is obvious that no great number of tentacles could start 
from the water-vascular ring itself, nor would their concen- 
tration there allow of any great increase in bulk in the body as 
a whole, and this seems to have led, from a very early stage, 
to the development of radial vessels, which may be described 
as elongations of the bases of the five primary tentacles, from 
which the paired tentacles sprang; and since these vessels 
would have had no strength if they had grown out, like 
