?4 WILLIAM BATESON. 



of any number. For example^ while in the divisions Asteridse 

 and Asterinidse the prevailing number is again five, among the 

 Solasteride we find that the arms of Solaster may be 

 thirteen or nine (as in S. 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 

 course, 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 Theel, ' 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 

 Theel might easily be thought at first glance to be segmented 



