220 



THE EVOLUTION OF THE METAZOA 



Fig. 39. A few cases of renovation in the thecate hydroids. A, 

 Plumularia sp. with embedded hydrothecae as result of reiterated 

 renovations. B, Aglaophenia sp. with reduplicated hydrotheca; 

 C, Halecium sp. D, 'L.ictorella sp., E, Synthecium sp. with gonangia 

 as renovants. (A-B, after Hadzi, C-E, after Broch.) 



Such is the case with cladogony which I described (Hadzi, 

 1919 Fig. 40), i.e. the separation of twigs of the colonies 

 of Thecata which are transported by the sea current or which 

 become attached to a substratum in the vicinity of the mater- 

 nal colony. Something similar can be found in some Siphono- 

 phora which belong to the group Calycophora where whole 

 specially organized cormidia are separated from the maternal 

 colony and continue to live independently, producing sexual 

 individuals. Finally, we find only in Hydrozoa an asexual re- 

 production of cormi as such. 



The fact may not be completely irrelevant that, contrary to 

 what has been found in Scyphozoa whose medusae can never 

 reproduce asexually, this kind of reproduction does not appear 

 quite so rarely in hydromedusae where it is more frequent in 



