Evolution and Symmetry in the Order of the Sea-pens. 135 



conformity with other floating forms, would probably have shown a perfect 

 radial symmetry, and, from the ancestral history, it would have probably 

 had an outline like a top or pear. Without referring to certain changes of 

 internal structures, which it is much more difficult to understand, we may 

 suppose that, at the time this intermediate form was assuming the full 

 Pennatulid characters, it was in form not unlike a Cavernularia, and 

 succeeded, at times, in obtaining an insecure foothold in the sand. This 

 suggestion is supported by the fact that C. malabarica, as related by 

 Fowler,* is the only species of the order that has been found washed ashore 

 in great numbers after a storm. 



The habit and structure of the Pennatulid stock being thus established, 

 the further evolution followed the main lines of increased powers of deep, 

 rapid burrowing in the sand, accompanied by a completion of the develop- 

 ment of the mirscles and a gradual change to an almost complete bilateral 

 symmetry of the colony as a whole. 



This conception of the evolution of the Pennatulacea, which I have 

 ventured to bring forward, seems to me to give a satisfactory explanation 

 of two difficulties that are met with in the alternative hypothesis of a 

 bilaterally symmetrical ancestry. It is very difficult to understand why a 

 bilaterally symmetrical colony provided with an axis and with powers of 

 burrowing deeply in the sand should lose the axis and become radially 

 symmetrical. It has been suggested that the Veretillidae are degenerate, but 

 I cannot see that there is a shadow of evidence to support this view. They 

 are not parasitic, sedentary, nor cryptic in habit, and there is no reason for 

 supposing that in any structural characters they show signs of retrogressive 

 evolution. The only ground for the assertion is tliat they show greater 

 variation than the bilaterally symmetrical families, bat, although it is 

 undoubtedly a fact that degenerating organisms and structures are more 

 variable than others that are not degenerating, it does not follow that, 

 because organisms or structures are very variable, they are consequently 

 degenerate or degenerating. 



As I have attempted to show in the earlier part of this lecture, the great 

 range of variation seen in the radially symmeti'ical Pennatulacea is to be 

 associated not with the idea of degeneration, biit rather with their feeble 

 powers of movement and their radial symmetry. 



* Fowler, G. H., ' Proc. Zool. Soc.,' 1894, p. 376. 



