132 



Prof. S. J. Hickson. 



have been derived by degeneration on the one hand, and the higher 

 bilaterally symmetrical forms by differentiation and specialisation on the 

 other. In the earlier writings of Kolliker, Studer, Wilson, Jungersen, and 

 Bourne, the view is expressed that the Pennatulacea are derived from a 

 bilaterally symmetrical stock, and such genera as Bathyptilum or Protoptilum 

 are regarded as the most primitive genera of recent sea-pens. 



It is to Kiikenthal* that we owe the first definite suggestion that it is the 

 radially symmetrical Veretillidte that are the most primitive, and with his 

 view I am in agreement. 



The principal reasons in favour of Kiikenthal's view may be summarised as 

 follows : — 



(1) The general structure of the colony of the radially symmetrical 

 Pennatulacea is simpler than that of the bilaterally symmetrical ones. In 

 the latter we find the two kinds of zooids, autozooids and siphonozooids, 

 distributed all over the surface of the rachis without any definite arrange- 

 ment in rows or leaves. It is difficult to believe that, in the evolution of the 

 sea-pen from its Alcyonacean ancestry, there could have arisen by some 

 great mutation that very definite and orderly arrangement of these zooids on 

 each side of the rachis, that we find in such genera as Pennatula and 

 Scytalium, and still more difficult to conceive a reason for a subsequent 

 change of this arrangement unless accompanied by some very definite 

 change of habit. ■ 



(2) In passing through the series leading from radially to the bilaterally 

 symmetrical sea-pens, we find an increasing differentiation in the structure 

 of the colony. Such definite organs as the radial canals of Virgularia 

 and Osteocella, the specially differentiated zooids, which I have called 

 " mesozooids," of Pteroeides and some species of Pennatula, and that 

 peculiar type of spicule known as the three-flanged or " dreiflvigelig " 

 spicule, do not occur at all in the Veretillidse, nor in any known genus of 

 the other groups of Alcyonaria. 



(3) The axis of the higher Pennatulacea is a skeletal structure which has 

 no homology with any structures found in the other orders of Alcyonaria. 

 It must have arisen within the Pennatulid stem as an organ adapted to the 

 needs of a free colony, with an elaborate and powerful set of muscles. We 

 should expect to find, therefore, this structure to be better developed in the 

 higher forms with the more powerful muscles than in the more primitive 

 forms, in which the muscular system is not so well developed. If we 

 accept the view that tlie radially symmetrical forms are the most primitive, 

 this is exactly what we do find. As already mentioned, it is only in 



* Kiikenthal, W., ' Verhandl. VII. Internat. Zool. Congress. Graz,' p. 563 (1910). 



