Evolution and Symmetry in the Order of the Sea-pe7is. 115 



increase in power of movement (vagility), there has also been in all 

 probability a diminution in the number of adaptive forms, or, in other words, 

 a decrease in plastic variability. And the increase in power of movement is 

 accompanied by a change from a previous radial symmetry to a more and 

 more pronounced bilateral symmetry of the parts. 



Moreover, if we consider individual characters, we find that many of them, 

 which are extremely variable in the forms which show radial symmetry, 

 and are sedentary or show slight signs of movement, become rigid or less 

 variable in the more active forms with bilateral symmetry. Characters 

 which are determined in the former by external forces become genetic in the 

 latter. 



2. 0)1 Sijmmetry and Variation i/i, the Pennatulacea. 



The general statement that forms the substance of the introductory part of 

 this lecture, namely, that radially symmetrical forms of animals are more 

 variable than bilaterally symmetrical forms, is so difficult to prove absolutely, 

 owing to a variety of complications in special cases, each of which requires 

 separate consideration, that it would be of interest to discover whether in 

 any one order which affords examples of genera showing both kinds of 

 symmetry the radially symmetrical genera are more variable than those 

 with bilateral symmetry. AVe find such an order in the Pennatulacea, an 

 order which includes forms that are almost completely radially symmetrical, 

 such as Veretillum, and forms such as Pennatula, that are bilaterally 

 symmetrical. 



At the time of the publication of the 12th edition of Linnteus' ' Systema 

 Natura ' (1768), eight species of the sea-pens were known, seven being 

 described under the generic name Pennatula, and one (the deep-sea 

 Umbellula encrinus) under the name Vorticella. 



These species were included by Linnteus in his sub-class Zoophyta, a 

 group of organisms which, notwithstanding the researches of Peysonnell 

 and of John Ellis, were still regarded by Linnaeus to be partly of the nature 

 of animals and partly of the natui'e of plants : " Zoophyta composita 

 Animalcula, in bivio Animalium Vegetabiliumque constituta . . . Stirps 

 vegetans, metamorphosi transiens in florens Animal." 



From the time of Linn;iius and the final settlement of the Pennatulacea as 

 an order of the Alcyonaria in the animal kingdom, little progress was made 

 until the " Challenger " and subsequent scientific expeditions sent home 

 their deep-sea booty. For most of the sea-pens are found to be inhabitants 

 of deep or very deep sea -water, and it is only by the appliances carried by 

 the great national deep-sea expeditions that they can be captured in large 

 numbers and variety. The richest collection so far obtained is that of the 



