96 INVERTEBRATA CHAP. 



recalls the solid planula stage of other Coelenterates, the slit-like 

 spaces being comparable to the incipient absorption spaces which 

 are the first stage in the formation of the gastric cavity in other 

 Coelenterates. 



Perhaps we might go farther and find in the stage which succeeds 

 to this a similarity to Actinozoon development. In the development 

 of Actinozoa an ectodermal stomodaeum is also formed, and the 

 primary lobing of the enterocoelie sac by four folds in Beroe might 

 be compared to the outgrowth of taeniolae or septa in Actinozoa. 

 There would, however, remain an irreconcilable difference, viz., that 

 the first tentacles in Actinozoa sprout from the pouches, whereas 

 in Ctenophora they occur between them for in Ctenophora, as we 

 have seen, the tentacle pockets cause the formation of taeniolae and 

 therefore alternate with the pockets of the gut. 



Reverting, however, to the planula stage common to all Coelen- 

 terates, a comparison of the later history of this stage in the various 

 groups reveals the real relationship of the Ctenophora to the rest. 

 In Hydrozoa, Scyphozoa, and Actinozoa, the planula, after a brief 

 period of swimming, fixes itself by its aboral pole, which becomes 

 the root of the future colony ; but in Ctenophora the planula never 

 fixes itself but remains free-swimming and develops a complicated 

 sense-organ at the aboral pole. 



Hence, if all Coelenterata have sprung from a planula-like 

 ancestor, the Ctenophora must represent a branch which never deserted 

 the free-swimming life, and which in consequence must represent the 

 main stem of Coelenterates, while the other groups, though far more 

 abundant at the present day, must represent degenerate offshoots 

 of this stock. Of these we may suppose that the Actinozoa represent 

 a group which assumed a bottom life later than the rest, and in which, 

 consequently, evolution had gone farther, and a stomodaeum had 

 been formed. In accordance with this conclusion we find that 

 Ctenophora present resemblances to the larvae of the higher forms 

 in far greater degree than do other Coelenterata, for it is to be 

 expected that higher forms would arise from a dominant free- 

 swimming group rather than from a degenerate sessile one. 



EXPERIMENTAL EMBRYOLOGY OF COELENTERATA 



But though in their later larval life the Ctenophora retain many 

 primitive features, in their earlier embryonic life they have undergone 

 great specialization. This will be made clear by contrasting the 

 results obtained by Zoja (1895-1896), who experimented with the 

 eggs of Hydromedusae, and those obtained by Driesch (1895) and 

 Fischel (1897-1898), who experimented with the eggs of Beroe. 



Zoja worked with the genera Liriope, Geryonia, Mitrocoma, Clytia, 

 and Laodice, and he separated the first blastomeres of the segmenting 

 egg with a needle. In the case of Clytia and Laodice he found that 

 a single blastomere of the 16-cell stage was capable of developing 



