118 



Prof. S. J. Hickson. 



symmetrical forms are more rapid and more powerful than the movements of 

 the radially symmetrical forms ; but, judging from the arrangement and 

 development of the muscles, it seems very probable that they are. 



The difference in variability between the radially symmetrical and the 

 bilaterally symmetrical sea-pens may be seen in almost all the important 

 characters upon which the classification is based. 



The axis is present in all the bilaterally symmetrical Pennatulacea. In such 

 genera as Pennatula, Pteroeides and Scytalium for example every specimen 

 that has been examined has been provided with an axis extending from one 

 end of the colony to the other. Moreover, in all the specimens of Pennatula 

 and of Pteroeides that I have examined the axis is always circular in section, 

 tapering at each end to a fine point, and in all the specimens of Scytalium and 

 Funiculina the axis is four-sided (square with rounded angles in section). 

 There seem to be no variations from these characters in any species. In some 

 of the intermediate genera such as Umbellula and Virgularia, for example, the 

 axis is complete but is sometimes four-sided and sometimes circular in section. 

 In the classification recently suggested by Kiikenthal the species of the genus 

 Umbellula are arranged in groups according to the possession of a round or a 

 quadrangular axis. In the genus Virgularia most of the species that have 

 been described have a cylindrical axis, but in V. rumphii and in V. gustaviana 

 the axis is sometimes cylindrical and sometimes four-sided. In the 

 radially symmetrical Pennatulacea the axis varies greatly in length or 

 may be absent. In the genera Actinoptilum, Echinoptilum and Renilla no 

 trace of the axis has yet been found. Of these genera the last two named 

 show signs of incipient bilateral symmetry but are, nevertheless, more closely 

 related in their general anatomy to the Veretillidse than to the higher forms 

 of Pennatulacea. In the genus Lituaria one species (Z. mcstralasice) shows an 

 incomplete axis, in the other two species the axis is complete but, as the 

 number of specimens of these species that have been examined is as yet 

 very small, it is quite possible that there is considerable variation in this 

 respect in each of the three species. It is interesting to note too that in 

 L. phalloides, according to Fowler, the complete axis is associated with an 

 incipient bilateral symmetry. In the genus Veretillum the axis is very 

 variable. In two species ( V. malayense and V. tenue) the axis is said to 

 be complete, but as only five specimens have at present been examined, it 

 may be a variable character. In V. cynomoriuvi, of which a very large 

 number of specimens have been examined, the axis is incomplete and varies 

 considerably in length. 



In the genus Cavernularia, which is, perhaps, the most typically radially 

 symmetrical of all the sea-pens, the axis is extraordinarily variable. Of this 



