134 



Prof. S. J. Hickson. 



sedentary Alcyonarian colony or indirectly through a floating or drifting 

 stage. In either case the ancestry must have been radially symmetrical. 



It seems probable from the little knowledge we possess of the natural 

 history of the sea-pens that they are all capable of boring into the sand or 

 mud at the bottom of the sea by the muscular movement of the stalk, and the 

 difficulty of deriving them directly from the sedentary ancestry is that the 

 body wall of such Alcyonarians is not provided with muscles capable of any 

 such movements. There must have been between the absolutely sedentary 

 ancestry and the more active burrowing Pennatulid an intermediate stage 

 with some powers of muscular movement, and it may be suggested that this 

 stage was a floating or drifting colony. The transition from a sedentary to a 

 floating habit is not difficult to understand. The feeble musculature of the 

 endoderni of Alcyonarians, which is used for contracting the ccelenteric 

 cavities under certain unfavourable conditions, such as removal from the sea- 

 water or exposure at low tide, could readily be adapted to slow pulsations 

 sufficient to keep a colony afloat in running water and particularly so if it 

 were supplemented by the ciliary action of tlie ectoderm. Moreover, an 

 Alcyonarian showing a dimorphism of the zooids such as we find in 

 Sarcophytum and Anthomastus, in which a flow of water through an elaborate 

 plexus of canals in the substance of the colony is produced by the action 

 of the siphonozooids, would be a more favourable form for adaptation to 

 such a pelagic mode of life than the heavier monomorphic forms. In 

 all the sedentary Alcyonaria about which we have information on the 

 point the cilia supported by the ectoderm cells of the larva are lost 

 when the fixation takes place. There is no record of a ciliated ectoderm 

 covering the colony in any species. In the Pennatulacea, on the other 

 hand, Kolliker* originally pointed out, there are certainly patches or 

 tracts of cilia on the ectoderm of the rachis, although they appear to be 

 absent on the stalk. My view, therefore, is that there was a stage in 

 the evolution of the Pennatulacea when the colony became free from 

 its sedentary habit and was dimorphic and ciliated. If this transition 

 actually occurred, it would not be exceptional in the animal kingdom. The 

 remarkable hydrozoan Pelago hydra, discovered by Dendy, was undoubtedly 

 derived from a sedentary ancestry, and there is good reason to believe that 

 the Salps and Doliolum were independently derived from sedentary 

 Tunicates. It is iinlikely that in this stage the axis was developed, as a 

 heavy skeletal structure of tliis character would be of little use for the 

 attachment of muscles such as would be used for pulsating movements, and 

 its weight would impair the flotation power. The shape of the body, in 

 * Kolliker, ' Anat. System. Beschreibung der Alcyonarian,' p. 424 (1872). 



