268 THE EVOLUTION OF THE METAZOA 



be essentially a larva; it rather represents a special generation, 

 it is an adult individual which belongs to the same species 

 and which can secondarily completely replace the primary 

 polypean individual. 



We will add here some observations regarding the condi- 

 tions that exist among the Turbellaria in order to complete 

 in this way our study of the Cnidaria larvae. It can be stated 

 quite generally that the mobile Turbellaria which live in 

 benthos do not show an inclination to develop larvae. There 

 are only a few and usually bigger species of Polycladida w^hich 

 had evolved their own larvae (the so-called Miiller's larva); 

 yet these larvae have not reached a very high level of evolution 

 either in their organization or in the special larval devices. 

 This larva assumes another position, i.e. it hangs in free water 

 with its head turned upwards, and it forms a girdle of body 

 process (usually eight) whose margins are covered with strong- 

 er cilia. I cannot agree with Kaestner who tries to connect 

 this margin covered with cilia with the praeoral circlet of 

 ciha that can be found in the trochophore of Annelida, and 

 with the tuft of cilia that occurs on the vortex of larvae. It is 

 much more simple to suppose that larva of Polycladida has 

 phyletically nothing in common with the trochophore (actual- 

 ly we have here quite different larvae w^hich resemble a trocho- 

 phore!) of Annelida. The circlets of cilia and the terminal 

 tufts of the same are requisites that can be frequently found 

 among the primary planktonic larvae. 



Miiller's larva represents (this is not true for all the Miil- 

 ler's larvae) in all probability the starting point for a side 

 line of evolution which had finally led to the formation of 

 Ctenophora by way of an early sexual maturity and of a 

 continued life in plankton. Here we can observe a very interest- 

 ing case of the invention of a larva. The typical Ctenophora, 

 themselves planktonic animals, had no need of a special larva 

 for the distribution of their own species. Let us say that a 

 typical transitional stage only had developed in the ontogenies 

 of Ctenophora; its form and organization resemble closely 



