110 



Prof. S. J. Hickson. 



In comparing the animals that show a dominant radial symmetry and are 

 sedentary or drifting in habit with those that show a dominant bilateral 

 symmetry and are free and active in their movements, it may be observed 

 that the former are far more variable in external form and in the number 

 and arrangements of their organs than the latter. 



If we take such a form as Hydra as an example of a sedentary animal, or 

 Aurelia as an example of a drifting animal, with radial symmetry, it is found 

 that, within the limits of what is at present regarded as a simple species, 

 profound variations in the important organs are of frequent occurrence. In 

 Hydra the number of tentacles and the number and form of the gonads vary 

 within a wide range in any collection of the same species taken from the 

 same locality. In Aurelia* and in Eucopef there are numerous variations to 

 be found, when a large collection of specimens is examined, in the number 

 and arrangement of the gonads, the gastric pouches, the radial canals, the 

 sense organs, and the tentacles, and if we add to these variations in the 

 important organs the minor variations in size, colour, length, or size of the 

 parts, etc., which are reckoned of some importance in systematic description, 

 the variability of these forms is found to be of great importance. If we 

 compare the extent of these variations with the variations seen in a typical 

 bilaterally symmetrical animal such as an earthworm or a crayfish a great 

 difference is observed. Thousands of specimens of a species of earthworm 

 may be examined without finding any variations in the number or position of 

 the gonads, of the oesophageal pouches, or of the principal blood-vessels. 

 Similarly, variations in the number or form of the appendages of a crayfish 

 are very rarely met with, and major variations in the viscera are so rare that 

 only a few isolated cases have been recorded. 



Many other examples could be quoted to illustrate this point of difference 

 between radially symmetrical and bilaterally symmetrical animals, but a 

 great many instances could also be brought forward which seem to be 

 exceptions to what we might otherwise call a biological rule. 



I have frequently been impressed, in my systematic work on Coelenterata, 

 with instances of the rigidity of certain characters in some species or genera 

 or even larger groups of these radially symmetrical organisms. The colour 

 of the spicules of Tuhipora musica is always red, there is always a blue coloiir 

 in the corallum of Heliopora, the spicules of Eunicella are always torch- 

 shaped, Paragorgia arhorea is always dimorphic, there are always eight 

 mesenteries in the Alcyonarian polyps and never more than eight tentacles. 



These characters, which in other genera and orders are very variable 

 * Ehrenberg, ' Ahh. k. Ak. Wiss.,' Berlin, 1835. 



t Agassiz and Woodworth, ' Bull. Mus. Comp. Zool.,' Cambridge, 1896. 



