XVII] THE COMPARISON OF RELATED FORMS 1059 



The hydroid zoophytes constitute a "polymorphic" group, within 

 which a vast number of species have already been distinguished; 

 and the labours of the systematic naturahst are constantly adding 

 to the number. The specific distinctions are for the most part 

 based, not upon characters directly presented by the living animal, 

 but upon the .form, size and arrangement of the little cups, or 

 "calycles," secreted and inhabited by the little individual polyps 



Fig. 514. 1, Harpinia plumosa Kr.; 2, Stegocephalus infiatus Kr.; 

 3, Hyperia galha. 



which compose the compound organism. The variations, which are 

 apparently infinite, of these conformations are easily seen to be 

 a question of relative magnitudes, and are capable of complete 

 expression, sometimes by very simple, 'sometimes by somewhat more 

 complex, coordinate networks. 



For instance, the varying shapes of the simple wineglass-shaped 

 cups of the Campanularidae are at once sufficiently represented and 

 compared by means of simple Cartesian coordinates (Fig. 515). In 

 the two allied famihes of Plumulariidae and Aglaopheniidae the 



