11 



GNIDARIA—REPBOBVCTION 



107 



(monodise Stpobila) the disc of the Scyphistoma {Ephyra) constricts 

 itself and separates from the peduncle, on which by regeneration a new 

 disc is afterwards formed. New discs, however, 

 are mostly formed between the peduncle and 

 the older discs before the latter detach them- 

 selves ; then we have the typical polydise 

 Strobila (Fig. 81). 



In Corals, reproduction by gemmation and 

 by incomplete fission is very wide spread. It 

 is, however, rare in the naked Actinia. It leads 

 to the formation of those occasionally very 

 large Coral-stocks whose skeletons are well known 

 as Reef or Stone Coral, the Gwal of commerce and 

 other Alcyonaria. The gemmation and stock 

 formation of the Alcyonaria is the most fully in- 

 vestigated. At certain points of the mother-polyj) 

 outgrowths, the so-called stolons, make their 

 appearance and are arranged on the mother- 

 polyp in ways characteristic of each different 

 group. They are either simple, or branched in 

 a reticular manner. By new outgrowths, and 

 local widenings of the endodermal canals which 

 they contain, there arise, on these stolons, 

 young daughter animals, in which the mouth, 

 oesophageal tube, septa, and tentacles are formed. 



Fig. si.— Polydise Strobila 

 of Aurelia aurita, after 

 Haeckel. 



In this way arise 



Fig. 82.— Diagrams Illustrative of gemmation and stock formation in various Alcyonaria. 

 A, General diagram. B, Gorgonia. C, Tubipora. D, Alcyonium. Black portions the cavities of the 

 gastral system, s, CEsophagus ; se, septa ; mf, mesenterial thickenings ; dh, gastric cavity ; sk, axial 

 skeleton, drawn in layers to illustrate the manner of its origin. 



