1IYDEOZOA.. 



237 



be separated as an independent layer of nucleated fibre cells below 

 the epithelium. 



The Polyps are not invariably alike, proliferous Polyps (or 

 Blastostyles) being frequently found as well as the nutritive ones. 

 The proliferous Polyps develop generative buds on their walls. The 

 sterile Polyps may differ from one another in the number of tentacles 

 and in their entire form, so that different kinds of individuals may 

 be found on a single stock. Thus we find the polymorphism of the 

 Siphonophora foreshadowed amongst the Hydroidea (Podocoryne^ 

 Plu'/nularia). 



The generative products are only exceptionally developed in the 

 Polyp body itself, in 

 which case they are 

 produced in the ecto- 

 derm (Hydra). This 

 exception is probably 

 to be looked upon as 

 an extreme case of 

 degeneration of a 

 medusoid bud. As a 

 rule the generative 

 products are de- 

 veloped in special 

 medusoid buds [gono- 

 phores] formed from 

 both cell-layers. 



In the most simple 

 cases the budding in- 

 dividuals of the sexual 

 generation contain a 

 diverticuluin of the 

 gastric cavity of the 



polyp-shaped parent or of the axial cavity of the hydroid stock. The 

 generative products become accumulated around this diverticulum 

 (Hydractinia echinata, Clava squamata). In a more advanced 

 stage we find a mantle-like envelope enclosing the bud, and con- 

 stituting the rudiment of the umbrella, with a continuous vascular 

 lamella or with more or less developed radial vessels (Tubularia 

 coronata, Eudendrium ramosum, Van Ben.) Finally, at the highest 

 stage, the buds develop into small M edusse (Campanularia gelatinosa 

 van Ben., Sarsia tubulosa), which become free, and sooner or later, 



FIG. 181. Podocoryne cornea (after C. Grobben). P, Polyp ; 

 M, Medusa bud on the proliferating polyp ; 8, spiral- 

 zooid; Sk, skeleton Polyp (compare the free Medusa, 

 fig. 154). 



