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of both hydrocoeles project freely as tube-feet, but on the right 
side as well as on the left evaginations of the external 
coelomic wall are formed, which give rise to perihaemal 
pockets. Therefore here, too, under the stimulus of an 
abnormally developed right hydroccele, the right posterior 
coelom has acquired powers of which, under ordinary circum- 
stances it appears to be totally destitute. Now the perihaemal 
pockets of an Asteroid are homologous with the dental pockets 
of an Echinoid, but in an Asteroid these pockets never form 
teeth in their interior, but give rise to the so-called external 
perihaemal ring. If we regard Echinoids as developed from 
Asteroids, as in my paper on Echinus esculentus (5) I 
have given reason for doing, then the evolution of an Echinoid 
from an Asteroid must have consisted to a large extent of an 
alteration in the chemical constitution of the embryonic 
tissues, so that these have come to react differently to the 
stimulus exerted on them by the hydrocoele. No doubt the 
hydrocoele itself has been also altered, so that the stimulus 
emanating from it is different in the two cases. 
Now a general review of the structure of the simplest and 
oldest stalked Echinodermata known has led Bather (2) to the 
view that the body of an Echinoderm was not at first 
dominated by radial symmetry, but only became so secondarily 
when the appendages of the hydrocoele had attained a radially 
symmetrical arrangement. The early palaeontological history 
of free Echinoderms (Eleutherozoa) is still unknown, but 
there is no reason to believe that it differed in this respect 
from that of fixed Echinoderms (Pelmatozoa) . We thus 
arrive at the interesting conclusion that the influence of the 
hydrocoele on the other tissues, which, in the development of 
the race, has led to the formation of the beautifully symmetrical 
Crinoid out of the somewhat shapeless sac-like primitive 
Pelmatozoon, plays an equally important part in the fashioning 
of the individual out of the undifferentiated embryonic tissues. 
Driesch (3) , as is well known, attributes the regulation of indivi- 
dual development to an indwelling “entelechy,” a rudimentary 
“ knowing and willing, ” which, out of the materials presented 
