CNIDARIA AS THE ONLY COELENTERATA 35 



types which he wishes to compare; but this must be done 

 cum grano salts. 



Even those peculiarities of different ontogenetic stages for 

 which the unsuitable name of Caenogeneses (foreign or 

 tampered peculiarities) has been invented by Haeckel, could 

 be profitably used. Yet in this connection it is necessary to 

 be even more cautious. I think that at this point morphologists 

 have gone much too far, especially when they tried to develop 

 the classification of animal types into large units on the basis 

 of similarities that can only be observed during the earlier 

 ontogenetic stages, e.g. during the segmentation of the egg, 

 during the formation of the entoderm, etc. To demonst- 

 rate the above statement we may take the systematic category 

 of Spiralia. A special type of cleavage, of the forma- 

 tion of the blastomere— the spiral type— is common to all 

 animal types (this of course is not without exceptions) even 

 if they show great varieties in their structures. Will this 

 type of cleavage prove the common origin of all the animal 

 species in which it appears, and should we therefore in a 

 natural system comibine all of them into one large group? 

 I completely agree with Steinbock's suggestion to look upon 

 these types of division from the point of view of the evolu- 

 tionary mechanics (I like to speak in this connection about 

 an "ontogenetic technique"). The type of cleavage usually 

 depends on the quantity of the reserve food and so it appears 

 in various forms within the narrow frame of a systematic 

 category. Sacarao (1952) has described many examples in 

 this connection which he explained in the same way. The 

 phylogenist finds it useless to speak about Spiralia because 

 among these there are whole groups that are no "Spiralia" 

 whatever. 



A special treatment is needed in the case of those ontogene- 

 tic stages that live freely and— as larvae— have had a progres- 

 sive development which has been, say, in other surroundings 

 and with another way of life than the corresponding grown-up 

 forms. These cases are important for phylogenetic purposes. 



