14 THE EVOLUTION OF THE META20A 



The difference between the tentacles of Ctenophora and 

 Cnidaria \\ould be perfectly clear if we could show the origin 

 of tentacles in Ctenophora as well as in Cnidaria. In both cases 

 the tentacles are, in all probability, not completely new for- 

 mations. We presuppose that in both cases the tentacles have 

 developed out of paired excrescences at the anterior part of 

 the body (at the "head") of their turbellarian ancestors and 

 that on both occasions this development was independent from 

 each other. It should be mentioned in this connection that 

 paired organs of touch (this in connection with the bilateral 

 symmetry of the animals) can already be found among Turbel- 

 laria, once with the inclusion of the intestinal diverticulum, 

 another time without it. Cnidaria must therefore have descended 

 from those turbellarian ancestors whose palpi had an intestinal 

 diverticulum, and Ctenophora from those whose palpi had 

 none. We can see that in Temnocephala, i.e. in Turbellaria 

 which have adopted a half-sessile way of life, the polymeri- 

 zation of these excrescences has already taken place; this has 

 become even more developed in Cnidaria which have similarly 

 adopted first a half-sessile and later a completely sessile way of 

 life. Ctenophora, however, as freely moving animals, retained 

 the original paired system of tentacles which became specialized 

 in another direction. In the first case (Cnidaria) the turbellarian 

 ancestors belonged to a rhabdocoelian subtype while in the 

 second case (Ctenophora) they belonged to the subtype of 

 Polycladida, and especially to their larvae that became neotenic 

 and remained in the plankton during the whole of their life. 



In this connection I would like to point to the fact that 

 in the course of phylogeny, tentacles have been repeatedly 

 evolved and have either a horseshoe-shaped (the younger phy- 

 letic stage) or crown-shaped form. This pattern has always 

 stood in connection with the transition to a sessile way of life 

 (Entoprocta among the Ameria, in nearly all Oligomeria, and 

 even in the "lower" Chordata: in Tunicata). 



The third peculiarity and at the same time the third point in 

 which Ctenophora differ sharply from the Cnidarian medusae 



