66 THE EVOLUTION OF THE METAZOA 



have really the so-called kleptocnidae, i.e. cnidae which have 

 been taken over from Cnidaria which had been devoured by 

 MoUusca— a certainly very remarkable case. It is very probable 

 that Nemertinea, too, possess kleptocnidae. A special place 

 among all the animal groups that possess cnidoids is occupied 

 by Infusoria-Turbellaria-Cnidaria; to these we must perhaps 

 add Nemertinea. It has been with intent that Ctenophora have 

 been omitted here, even if the presence of genuine cnidae can 

 be frequently found mentioned in the special literature in con- 

 nection with at least one species of Ctenophora (Haeckelia or 

 Euchlora rubra). As has been mentioned earlier, I think that 

 it is very probable (Hadzi, 1951) that only kleptocnidae occur 

 in this ctenophore and that they have been taken over from 

 a species of Narcomedusae which serve as the main food of 

 this ctenophore. 



There are, besides the typical cnidae, some less specialized 

 forms that occur in the large groups of animals which have 

 been mentioned above. They are essentially glandular; their 

 secretion is partly solid, and partly a fluid that can absorb 

 water and swell. They have been described under various names 

 (trichocysts, rhabdites, sagittocysts). 



The form of medusa must be considered as a kind of parallel- 

 ism; it is neither homologous, nor analogous, nor a result of 

 convergences (see Fig. 13). We shall omit here those Protista 

 which show only a purely external similarity with medusae 

 (among Flagellata e.g. Medusochloris, Craspedotella, and I^epto- 

 discus) ; these have been called the "types of the forms of life" 

 by Remane (1944). The genuine medusae must be considered 

 to have evolved beyond any doubt at least twice, independently 

 from each other; both times they have developed from a 

 polyp by way of an adoption of the vagile way of life: the first 

 time from a scyphopolyp, and the second time from a hydro- 

 polyp. I will show later that this hydropolyp had been deve- 

 loped later than the scyphopolyp, and in all probability from 

 just this same scyphopolyp. A similar way of life (natural se- 

 lection!) and a close enough relationship (the same direction 



