206 THE EVOLUTION OF THE METAJ'.OA 



developed simultaneously with Metchnikoff the planula 

 theory. 



It is characteristic that so far there has been nobody who 

 prepared to interpret the ontogeny of Hydra as typical in 

 spite of the fact that until recently Hydra has been conside- 

 red as the most primitive cnidarian (even now we can find in 

 the recent book by Hyman the family Hydridae given the 

 first place in the system of Hydrozoa, a classification which 

 must presuppose these to be the most primitive Hydroidea 

 and therefore also the most primitive forms of Cnidaria). A. A. 

 Sachwatkin (1956), also tried to derive Metazoa (together 

 with sponges) from the colonies of Flagellata. He considers 

 the flagellate blastula to be the primitive form, and corres- 

 pondingly had to bring forward the "metagenetic Leptolida" 

 in order to demonstrate his ideas in one example of the 

 supposedly primitive Metazoa. The examples selected by 

 Sachwatkin, i.e. the calcareous sponges, the metagenetic 

 Leptolida, the "lower Scyphozoa," and sea urchins represent 

 a poorly selected group because they all belong, without 

 exception, to very specialized types, and because they cannot 

 actually even be compared with each other. 



In our analysis and comparison of ontogenetic morpho- 

 genies we will consequently begin with the study of conditions 

 as they occur in Anthozoa, above all in those that live a prima- 

 rily solitary life. These are the Actiniaria and Ceriantharia. We 

 find a "typical" ontogeny with a total cleavage, with a flagel- 

 late blastula, and with an invagination gastrula in Actiniaria, 

 among Anthozoa: this in spite of a presence of various vitelline 

 substances. Haeckel himself selected a species of anthopolyp 

 as a typical example (Fig. 37). As early as in Octocorallia 

 we find this situation changed. The segmentation, though 

 total and subequal, is followed by a stereoblastula; gastru- 

 lation takes place by way of a secondary delamination. Few 

 species only belonging to Scyphozoa have been studied so far 

 from this point of view (mainly Nausitho': and Chrysaora) . 

 Oeavage seems to be more unequal here, and its result is a 



