112 



Prof. S. J. Hickson. 



'symmetry as regards the individual of which the colony is composed, exhibit 

 a great variety of form as regards the colony as a vs^hole. 



The study of the form of animal colonies is one of the most difficult, but 

 at the same time most interesting, studies vsrithin the range of biological 

 science, but at present very little progress has been made, and the solution 

 of many of the problems that it affords is quite obscure. But vpe can see 

 that in the formation of full-grown colonies there is a prevalent tendency to 

 the construction of a radially symmetrical shape. Among the Madreporarian 

 corals, for example, we find many examples of the solidly constructed 

 Meandrinas, Siderastrteas, etc., and of the ramose Pocilloporas and Seriato- 

 poras that are spherical or hemispherical in shape. Among the Alcyonaria 

 we find the club-shaped Alcyonium colonies, the mushroom-shaped Sarco- 

 phytums, the cylindrical Juncellas and the symmetrical bush-shaped colonies 

 of such as species as Chrysogorgia flexilis. Among the sponges we find the 

 spherical masses of Suberites, Tuberella and Tethea, and the horn-shaped 

 Euplectella. Among the Polyzoa we find the beautiful funnel-shaped 

 colonies of Ketepora, the spherical masses of Cellepora and the disc-shaped 

 colonies of Lagenipora. 



In many other examples, however, the colony is not strictly radial in 

 symmetry, as, for example, the flabellate forms of Gorgonia, the plicate forms 

 of Madrepora and Millepora, and countless other varieties of branching and 

 encrusting growth. 



The general principle that seems to underlie the growth of these colonies 

 is a tendency to assume a radially symmetrical form subject to modifications 

 within a wide range, in adaptation to the variable conditions of the environ- 

 ment. 



Thus, if the larva of one of these organisms settles down in shallow water 

 in an open space where the necessary food may drift towards it from any 

 point of the compass, it will, in the formation of its colony, send out branches 

 of equal length in all directions, and thus assume a spherical shape ; if it 

 settles down in a narrow space between two massive rocks or corals, where 

 the food from two directions is cut off, the shape of the colony will tend to 

 become oval or flat. If the larva settles on a spot where the sea currents or 

 wave actions are strong the branches will be strong and tend to anastomose; 

 if it settles in deeper water where such are less powerful, the branches grow 

 more delicately and do not anastomose. An interesting variety of such 

 adaptation to conditions is seen in the genus Stylaster, one of the genera 

 of hydroid corals that extends from shallow water to great depths of the 

 ocean. The general form assumed by colonies of this genus is that of a fan, 

 the main branches radiating from the base in one plane give rise to a large 



