114 



Prof. S. J. Hickson. 



the active bilaterally symmetrical genera of the animal kingdom, and minor 

 variations of shape cannot be used with any confidence for the diagnosis of 

 species. 



In the building up of the colonial organisation, the various methods of 

 growth bring the individual polyps into a position in the water where they 

 will receive during their vigorous lifetime the most abundant supply of food 

 and, in some cases, of light. In such positions the food upon which the 

 polyps feed will usually come, not from one direction only, but with greater 

 or less frequency from all directions, and it is therefore to their advantage 

 that the tentacles, of approximately equal length, should radiate from a 

 common centre, where the mouth is situated. Upon the arrangement of the 

 tentacles the whole symmetry of the body depends, and it is found that, 

 whatever shape the colony as a whole may have assumed in adaptation to its 

 environment, the radial symmetry of the tentacles, of the hard protective 

 skeletal structures of these polyps, and of certain other organs of the body, is 

 maintained. 



But although the structure of the individual polyp and of the hard parts, 

 such as the pores, calices, septa, cells (of Polyzoa), etc., are rightly regarded 

 by systematists as more reliable than the form of the colony for the 

 diagnosis of species, even these are liable to considerable variation in detail, 

 according to their position on the colony. 



The great variability that is observed in most of the species of colony- 

 forming animals may be associated with their sedentary or stationary habit of 

 life. It is only in a few cases, such as Pelagohydra, Cristatella, the Salps, 

 and a few others that we find that the colony as a whole has the power of 

 moving by its own efforts from place to place. In the order of the sea-pens, 

 if we may judge from the character of the muscles, and from the direct 

 evidence of observation of a few genera, every colony has the power of 

 forcing its way into the sand by its own muscular movements. It seems 

 probable that their movements are more powerful and rapid than the 

 movements of any other colony of animal zooids. 



Now, as Doderlein* has pointed out, the power of movement is correlated 

 with plastic variability. When a species for any reason becomes less active 

 in its movements — and he quotes Bosmina coregoni and Daphnia hyalina 

 among others as examples of this — it tends to become much more variable. 

 His view is summarised in the statement : " Die Hohe des Vagilitat bei 

 verschiedenen Thiergruppen muss in umgekehrtem Verhaltniss Stehen zur 

 Zahl der auf dem gleichen Gebiete vorhandenen adaptiven Formen." 



Similarly, if in the evolution of a group of animals there has been an 

 * Doderlein, L. ' Zeitschr. fur Morphologie,' vol. 4, p. 441 (1902). 



