70 



THE EVOLUTION OF THE METAZOA 



natural that in the parents the old characteristics will strong- 

 ly prevail, and in the descendants the new characteristics which 

 are distinctive for the new type. 



We shall find without any difficulty, if we limit ourselves to 

 the group Cnidaria-Turbellaria-Ctenophora, and if we make 

 a comparison of their recent forms, taking simultaneously into 



Fig. 14. Three species of Temnocephala; illustrating the poly- 

 merization of tentacles like excrescences. A, Bubalocerus pretneri 

 (from Matjasic). B, Temnocephala chilensis (after Wacke). C, Aciino- 

 dactylus hlanchardi (after Haswell). 



consideration Watson's rule, that the validity of this rule ex- 

 tends also to the lower invertebrates, and, consequently, over 

 all the animal groups. Here we shall limit ourselves to mention 

 a few characteristic examples only. Thus we can find in various 

 Turbellaria: (1) the body set upright while mobility is at 

 the same time temporarily preserved (this led to a complete 

 sessility and immobility, to the formation of cormi and of 

 colonies, and finally to a polymorphism connected with the 

 formation of medusae); (2) the tentacle-like excrescences at 

 their anterior ends which show an inclination to polymeriza- 

 tion (Temnocephala, see Fig. 14), and the growth of the intes- 

 tinal diverticula into the interior of these tentacles (in polyps 



