CNIDARIA AS THE ONLY COELENTERA.TA 37 



life of the adult stage can become very short: particularly 

 in the case of male animals, so short that it just suffices for 

 propagation. In these instances, the differences between 

 a larva and its imago have become so considerable, both 

 with regard to their structures as well as their ways of 

 life (the type of feeding, the sphere within which the animal 

 lives) that finally a great crisis occurs and the ontogeny 

 which can only be overcome by way of an extremely radical 

 method i.e. that of the total metamorphosis. These instan- 

 ces have the character of true catastrophies which, however, 

 can fortunately enough be mastered. In the case of strongly 

 developed parasitism we can frequently observe that a larva 

 which has preserved a free mode of life, reaches a higher 

 level of development than its grown-up form. 



Beginning with Turbellaria and ending with Amphibia 

 among the Vertebrata, larvae have had an important role du- 

 ring the levels reached in evolution. It is therefore not surp- 

 rising that attempts have been made to use larvae as a basic 

 element in the construction of a natural (i.e. evolutionistic) 

 animal system. This is particularly true for those zoologists 

 who came under the influence of the supposed "fundamen- 

 tal biogenetic law" as it had been formulated by Haeckel. 

 It was all too easy to accept the possibility that in larvae, 

 an ancestral form is being recapitulated and that for this 

 reason all smaller groups whose larvae show a certain degree 

 of similarity must have (1) a common ancestor, and (2) the 

 form of this common ancestor must have been basically 

 the same as that of the recent larva. In spite of the fact 

 that these theses have been found to be essentially untrue, 

 they have proved useful in the struggle for the recognition 

 of the theory of evolution. This particularly in the sense that 

 they have numerous well-known zoologists to make "emb- 

 ryological" studies of such species and groups of animals 

 where natural relationships are not clear. It had been hoped 

 that the better known ontogenies could prove useful in 

 attempts to unravel these connections. The facts which have 



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