PREVIOUS INTERPRETATIONS OF CNIDARIA 213 



at all? An illuminating example in this connection is the athe- 

 cate hydroid Turritopsis nutricola whose ontogeny begins with 

 a regular cleavage that soon becomes irregular in order 

 finally to lead to a syncytial state; naturally enough this deve- 

 lops later into a cellular state. Even in other cases in the Cnida- 

 ria, a return to a syncytial state is not at all rare. 



As a conclusion we can say that the facts that have been 

 known so far about the ontogeny of Cnidaria and Ctenophora 

 do not stand in opposition to our concept. On the contrary, 

 we find these facts if viewed from our standpoint are more 

 understandable than they have been till now. Two trends can 

 be found in ontogeny (as in the morphology) of the adult 

 forms: a retrogressive development, and a progressive trend. 

 The retrogressive evolution which takes place parallel to the 

 simplification of structure leads to a reduction of the 

 middle layer and to the in-determined character of the early 

 development, and the progressive trend leads to the formation 

 of a planula larva typical of Cnidaria. Both these developments 

 depend on the sessile way of life, combined with the formation 

 of cormi. 



Asexual Reproduction and Regeneration 



No phylogenetic significance has usually been attributed to 

 asexual reproduction. It is usually considered that where 

 the sessile way of life has been adopted by Eumetazoa it 

 appeared spontaneously. This is generally true even if there 

 are numerous exceptions. It should suffice to mention here, 

 on one hand. Rotatoria, Gastropoda, and Lamellibranchiata 

 which incline to partial sessility, and the completely sessile 

 Brachiopoda: among all these animals not a single case of 

 asexual reproduction has been known so far; and, on the other 

 hand, Annelida which move freely and which show, in spite 

 of this, instances of an asexual reproduction not only by 

 means of division but also by way of budding. In all those 

 cases where freely moving animals reproduce asexually we 

 15 



