Evolution and Sy mmetry in the Order of the Sea-pens. 113 



number of delicate branches which fill in the spaces between the main radii 

 and frequently anastomose. As in all shallow water corals, however, the form 

 of the colony is subject to great variations. 



The polyps of Stylaster are arranged in cycles of five or six individuals — 

 the Dactylozooids — with one larger individual — the Gasterozooid — in the 

 centre of the cycle, and each of these cycles is protected by a calcareous cup 

 which is called the calyx. 



In the shallow-water species, such as Stylaster eximius, the calicos are 

 situated on the sides of the branches, that is to say, with the mouths of the 

 cups facing in a direction parallel with the plane of the fiabellum. In some 

 of the deep-sea species, such as S. umbonatus and S. viinimus, the calices are 

 all situated on one surface of the branches, that is to say, with the mouths of 

 the cups facing in a direction at right angles to the surface of the fiabellum. 

 It seems probable that the former arrangement is an adaptation to life in the 

 strong varying currents and wave action of shallow water, and the latter to 

 life in comparatively quiet water, with a gentle current fiowing mainly in 

 one direction, such as we find in some localities in deep water. 



It is not necessary to refer to further examples of the great variability of 

 external form, mode of growth, and method of ramification of the sedentary 

 colonies of animals. They are well known to systematists interested in the 

 various orders of the Porifera, Coelenterata, Polyzoa, and Tunicata, but it 

 must be remarked that the range of variability is by no means the same in 

 all cases. In some genera, such as Millepora, the range is very wide. We 

 find flat encrusting forms, plicate forms, fan-shaped branching forms, forms 

 with thick anastomosing branches, and forms with delicate, not anastomosing, 

 branches, of shrubby aspect. The anatomical structures of the polyps rtiat 

 build up all these manifold kinds of colony are found, on careful examination, 

 to be identical. 



In other genera, such as Meandrina, solid more or less spherical or oval 

 colonies only are found ; in others, such as Juncella, long cylindrical 

 colonies ; in others, such as Seriatopora, branching bushy colonies ; and in 

 many other genera fiat laminate colonies following the contours of their 

 support. 



The shape assumed by a full-grown colony is not, therefore, entirely due to 

 individual adaptation to the surrounding conditions. There is undoubtedly 

 a genetic influence tending to produce a type of growth for each genus of a 

 family, but this influence allows departures from the exact model of the 

 type within certain ranges, which vary in extent in different genera. 



Allowing for differences of this kind in the plasticity of growth, however, 

 all the colonial and sedentary genera exhibit greater variation of shape than 



