12 HlCKSON, Animal Symmetry. 



Bernard 2 has already come to a similar conclusion as 

 regards certain genera of Madreporaria, and his view is 

 supported by some interesting experimental work by 

 Wood Jones. :J I may also call attention to the valuable 

 work by Matthai 4 on the corals of the family Astraeidae, 

 in which, by a comparison of the structure of the polyps 

 with that of the hard skeletal parts, he has shown that a 

 very considerable reduction in the number of species 

 attributed to several genera should be made. 



Many years will probably pass before zoologists arrive 

 at a common agreement to abandon the present system of 

 making species in these groups, but the results of all 

 recent investigators point irresistably to the conclusion 

 that whatever limit we make to the extension of a specific 

 group, the species of these sedentary colonies are far more 

 variable than the species of the free bilaterally symmetri- 

 cal animals. A question of more general interest however 

 is whether there is sufficient evidence that the difference 

 in variability of the species in a group of animals which 

 show a series of gradations from a radial to a bilateral 

 symmetry. We have such a group in the Sea-pens or 

 Pennatulacea and it has been my good fortune to have an 

 opportunity of studying, in detail specimens of nearly all 

 the known genera and a large proportion of the known 

 species. 



The Alcyonaria belonging to the Order Pennatulacea 

 form colonies which are not attached to stones or rocks, 

 but are free and capable of a certain amount of movement 

 in the mud at the bottom of the sea. As nearly all the 

 Sea-pens are found in deep water we know very little 



- Bernard, H. M. Proc. Camb. Phil. Soc, 1901, xi., p. 268; and 

 British Museum Catalogues, Madreporaria, Vol. IV., 1903. 



3 Jones, F. Wood. Proc. Zool. Soc, 1907, p. 518. 



4 Matthai, G. Trans. Linn. Soc., XVII., 1914. 



