542 WILLIAM BATESON. 
of any number. For example, while in the divisions Asteridz 
and Asterinide the prevailing number is again five, among the 
Solasteride we find that the arms of Solaster may be 
thirteen or nine (as in 8S. endeca), in Heliaster from twenty- 
nine to forty. Not only is this true of living forms, but in 
the case of the fossil Cystidea the plates were irregularly 
arranged and the perforations of the feet scattered, and in the 
Blastoidea the basal plates were three, though bearing ~ 
five radials and interradials. All these facts point to a 
history of the occurrence of repetitions among the various 
parts around a central axis. And perhaps more remarkable 
still is the extreme variability to be seen among individual 
members of living species. 
For example, though Asterias rubens ordinarily possesses 
five arms specimens possessing six or seven arms are very 
common, while individuals with only four are not rare (the 
latter may possibly, however, arise from mutilation). In like 
manner specimens of Brisinga coronata are said to have 
from nine to twelve arms. Thus, in these cases the arms, with 
all the organs which they contain, may be spasmodically 
repeated as a mere individual variation. 
All these animals move on the oral surface, and though, of 
ceurse, the body may be regarded as arranged bilaterally 
round a longitudinal axis, yet in the locomotion of the animal 
this fact is not conspicuous (?) But in the Holothurians in 
which a long axis does again assume importance, though 
repetitions of this magnitude do not occur, yet there is a 
tendency for certain organs to arrange themselves in a series 
of longitudinal repetitions closely imitating segmentation. 
In this connection the Elasipoda (Holma Théel, ‘ Challenger 
Monographs’), which crawl about on the “ trivial”’ surface in 
the direction of the long axis are of great interest. The body 
of these animals is long and flat, and its margins are produced 
into long processes, resembling parapodia, which are regularly 
arranged in pairs down the sides. The regularity of this 
arrangement is so great that some of the species figured by 
Théel might easily be thought at first glance to be segmented 
