92 



THE EVOLUTION OF THE METAZOA 



as buds to the latter (see Fig. 17); and, finally, they have comple- 

 tely disappeared when they have abandoned the state of 

 cormi (the form of a colony). This has been more than probably 

 the case with the freshwater Hydrae. 



Again and again the adherents to the idea of the primacy 

 of the medusa form refer to the hypogenetic trachvlinae 

 Hydromedusae(Narco- and Trachy-medusae) ; yet these schol- 



Fig. 17. Progressive reduction of sessile medusae (medusoids). 



A, B, Eumedusoids oiT iibularia and Campanularia. C,D,cryptomed- 



usoid (Gonothyrea ,Clava, etc.). E, heteromedusoid (Grammaria sp.). 



F, G, styloids (Riidendrium^ etc.). (After Kiihn and Broch.) 



ars are inconsistent because they do not place Trachylinae 

 in a classification, which is supposed to be natural, at the 

 beginning of Cnidaria, but rather— which is actually quite 

 correct— close to their end, usually as the penultimate group, 

 followed by Siphonophora, i.e. they classify them as one of 

 the most specialized groups (cf. the system proposed by 

 Hyman). 



The hypothesis of an actinula as a primitive form and as 

 an ancestor of Narcomedusae as well as of Hydroidea, and 

 thus of all Hydrozoa, and in the last line of all Cnidaria, had 



