428 THE EVOLUTION OF THE METAZOA 



of Species. They are the Ectoprocta and the Echinodermata. 

 All these species live without any exception in water, and 

 there are few exceptions only (among the Ectoprocta) which 

 do not live in the sea. With the exception of the class Chaeto- 

 gnatha— which is also otherwise an aberrant group— we find 

 that all the groups of the OHgomeria live on the sea bottom. 

 Lameere (1931) and Had^i (1958a) have shown that we can 

 best explain the organization and the place where the Chaeto- 

 gnatha live, which is quite unusual for the OHgomeria— the 

 free water zone— if we consider that the Chaetognatha evolved 

 by way of neoteny from the planktonic larvae ot the Brachio- 

 poda. There is no other class or even a lower systematic category 

 among the recent OHgomeria whose species have also become 

 adapted to a life in plankton. Individual species can only be 

 found among the Echinodermata which had also evolved into 

 planktonic animals {Velagothuria and a few other species), a fact 

 which could be least expected because of the internal skeleton 

 which had also been developed by these same animals. On the 

 other hand w^e find a rich variety of planktonic larvae developed 

 by these OHgomeria, a fact which could easily be expected; 

 these larvae show widely different forms and structures which 

 are of Httle use as material used in our phyletic speculations. 



The fact that among aH the recent nine classes of the OHgo- 

 meria there are only two with a richly developed asexual rep- 

 roduction combined with a formation of cormi (Ectoprocta 

 and Pterobranchia), shows the Hmited plasticity of the OHgo- 

 meria. Polymorphism can be observed quite generaUy among 

 the Ectoprocta. A very surprising circumstance is the fact that 

 even a dully sessile class such as the Brachiopoda are no 

 longer able to reproduce asexually. We can understand this if 

 we interpret the OHgomeria as secondarily simpHfied and 

 speciaHzed animals which had evolved from the Polymeria by 

 way of a retrogressive evolution. 



On the other hand, widely different trends in evolution can 

 be found within the group OHgomeria itself; the situation here 

 is very similar to the one we have been able to observe in the 



