xviij THE COMPARISON OF RELATED FORMS 747 



by the living animal, but upon the form, size and arrangement 

 of the little cups, or "calycles," secreted and inhabited by the 

 little individual polypes which compose the compound organism. 

 The variations, which are apparently infinite, of these conforma- 

 tions 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, co-ordinate 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 co-ordi- 

 nates (Fig. 371). In the two allied families of Plumulariidae and 



Fig. 371. a, Campanularia macroscyphus, Allm. ; b, Gonothyraea hyalina, 

 Hincks; c, Glytia Johnstoni, Alder. 



Aglaopheniidae the calycles are set unilaterally upon a jointed 

 stem, and small cup-like structures (holding rudimentary polypes) 

 are associated with the large calycles in definite number and 

 position. These small calyculi are variable in number, but in the 

 great majority of cases they accompany the large calycle in 

 groups of three — two standing by its upper border, and one, 

 which is especially variable in form and magnitude, lying at its 

 base. The stem is liable to flexure and, in a high degree, to 

 extension or compression; and these variations extend, often on 

 an exaggerated scale, to the related calycles. As a result we find 

 that we can draw various systems of curved or sinuous co-ordinates, 

 which express, all but completely, the configuration of the various 



