126 EMBRYOLOGY 



ontogeny does not i-epresent the primitive condition in this 

 regard. 



The attached polyp form recurs in the ontogeny of most 

 Cnidaria. In the Anthozoa and Lucernarida3 it constitutes 

 the adult animal ; in the Hydi'ozoa it is co-oi'dinate with 

 the Hydroraedusa ; whereas in the Acraspeda, in comparison 

 with the highly developed medusa form, it must instead be 

 considered as the young stage. Many medusa? (Tracheo- 

 medusfe, Pelagia) develop directly from the free-swimming 

 larvje into the medusa (comp. pp. 53 and 118). But here 

 also certain conditions of development can be interpreted 

 as modified polypoid stages. 



The development of free-swimming sexual forms 

 (medusog) did not take place until after the separation 

 into hydi^opolyps and scyphopolyps, and therefore occurred 

 in the two groups independently. The diiferences in or- 

 ganization between the hydroid- and scyphopolyps are 

 explained by the different structure of their polyp forms 

 and by their independent development. The hydroid- 

 medusa is developed as a lateral bud, whereas the strobila- 

 tion of the ScyphomedusiB is to be explained as a process of 

 transverse division. The medusa must be explained as a 

 polyp which acquired powers of free movement, and as a 

 result of this underwent certain changes in form. The first 

 cause for the evolution of such locomotion we have re- 

 cognized in the migration which in non-sexual reproduction 

 (division, budding) the detached portion must undertake 

 before attaching itself. 



An opposite explanation, which is based chiefly on the occurrence of 

 hypogenetic forms, and which sees in these the more primitive conditions, 

 starts from a free-swimming medusoid ancestral form, the larvje of 

 which, also at first leading a pelagic life, had secondarily acquired the 

 attached mode of life and reiJroduction by budding or division. The 

 polypoid forms would then have to be considered as coenogenetically 

 interpolated larval conditions (C. Vogt, No. 115 ; Brooks, No. 17). How- 

 ever, the entire structure of the medus» points to a primitive attached 

 ancestral form too clearly for us to grant this interpretation. 



In the search after those hypothetical free-swimming 

 ancestral forms which preceded the attached Hydra-like 

 form, we must first think of such creatures as are repre- 



