262 THE EVOLUTION OF THE METAZOA 



worst mistakes that have been committed in the history of 

 the comparative morphology and of the phylogenetics (and 

 which is, unfortunately enough, still made) has been the 

 attempt to use these changes in the ontogenies as a basis 

 for phylogenetic speculations and simply to transfer these 

 ontogenetic and morphogenetic processes into the phylogeny 

 of the adult forms. This has been done in spite of the numer- 

 ous cases where it is obviously impossible that an organ or 

 an orifice (e.g. the anal and oral orifices) had originally evolved 

 in the same \^ay as it is now^ developed in the ontogenies. 



I mention here as an example the pilidium of Nemertinea. 

 Who could think even for a moment that the "formation 

 of a worm" as it can be observed in connection with the 

 pilidium could be at the same time a recapitulation of an 

 ancestral form? Ho\\' can it be certain, or at least probable, 

 that we have a recapitulation when one and the same organ 

 or characteristic property can be found in such widely dif- 

 ferent forms in the ontogenies of closely related species, or 

 even (as has already been mentioned, see p. 21 1) in the ontogeny 

 of one and the same species; especially when it occurs in a 

 way w^hich cannot possibly be reconciled with the way of 

 life of the adult ancestral form (e.g. the coalescence of the so- 

 called primitive mouth, and the new formation of an anal 

 or oral orifice in the same place)? 



What is then the situation in connection with the larvae 

 of Cnidaria? We can state that no genuine and generally 

 occurring planktonic larva has actually been developed by 

 the Cnidaria (Fig. 46). The Planula which is the most fre- 

 quent form has usually been interpreted, as well as acti- 

 nula as the (ontogenetically and phylogenetically) "young- 

 er" larva. Besides these there are also som.e other larval forms, 

 the cerinula, the ratarula, and the siphonula which can be 

 met with within the limits of some smaller groups. In no 

 case can we find among Cnidaria a more highly developed 

 larval type whose subsequent development would be subject 

 to a metamorphosis as this is the case, even if to a small 



