NEW INTERPRETATION OF CNIDARIA 239 



The species (genera and families) with a more or less cylind- 

 rical or with a down-turned conic hydrotheca, with the for- 

 mation of medusae as individual buds, and without the theca 

 lid, should be placed at the beginning of the system of Hydro- 

 idea (e.g. Hebellidae). The subsequent evolution ot Thecata 

 diverged into several directions with a great variety in the 

 forms of thecae, with polymorphism, retrogression of the 

 medusa form, and with various forms of cormi which can 

 also include individualized forms (a secondary monopodial 

 growth). A special direction in evolution can be observed 

 in those species whose hydrotheca has been progressively re- 

 duced (fam. Haleciidae) so that finally these animals have 

 completely ceased to form a theca (fam. Campanopsidae). The 

 last remains of a former theca can be found in the so-called 

 tonofibrils which in the Thecata had provided an elastic connec- 

 tion between the basis of the theca and the hydranth (the so- 

 called luminous dots over the diaphragm which separate the 

 hydranth from its stem). This is how we can understand the 

 evolution of the Athecata from the Thecata. The possibility 

 cannot be completely excluded that the evolution from a thecate 

 to an athecate state did not occur once only, yet such a 

 polyphyletic development must be first specifically proved. 



This is not the right place to discuss all the details. It should 

 suffice that we give here in a broad outline, a new and more 

 suitable system of Hydrozoa, as it appears if we view Hydrozoa 

 as animals that have evolved from Scyphozoa. The old systems 

 which are mainly based on the actinula hypothesis and which 

 try to avoid comparisons between hydromedusae and scypho- 

 medusae must now be abandoned. The classification of Cnida- 

 ria must be necessarily based on the morphology of the main 

 primitive form which is comm^on to all Cnidaria, i.e. that of 

 the polyp. The hypogenetic species that can be found both 

 among Hydrozoa as well as among Scyphozoa, are a result of 

 a secondary development. This development, however, had 

 followed two different lines which can be understood only 

 if we take into consideration the way that medusae had evolved 



