328 THE EVOLUTION OF THE METAZOA 



observed in the ontogenies and under the assumption that they 

 represent some kind of recapitulation. The development of a 

 digestive organ by way of internalization, as was suggested 

 by the old interpretation, can in reality be taken into conside- 

 ration in connection with the Spongiae only ; yet even in this 

 case it did not lead to the formation of a genuine intestine. 



The old interpretation meets great, insurmountable, diffi- 

 culties when it tries to explain the subsequent evolution 

 of organs, thus of the protonephridia, of the muscular sys- 

 tem, of the nervous system, etc. All these "tasks" were 

 already solved on the level of Protozoa. The way to all these 

 differentiations had been prepared as early as in Infusoria. This 

 is true not only as regards the size of the body, its form, its 

 symmetrical conditions, the ciliation and the type of move- 

 ment, but also for the type of feeding, the transverse division, 

 the sexual phase together with the hermaphroditism and with 

 the pairing, the triploblastic system and for all the peculi- 

 arities of the three layers. 



The long list of similarities that can be found both in the 

 Infusoria and in the Turbellaria (a list which was originally 

 made by v. Gelei and which has now been enlarged) shows 

 that these are obviously no accidental similarities, convergen- 

 ces, or parallelisms; they are instead true homologies w^hich 

 exist because they go back to a common line of evolution. 

 These homologies are too numerous for us to consider them as 

 accidental. The progressive evolution which began from a 

 common basis continued especially as regards the middle body 

 layer. We will later return to discuss this problem in more 

 detail. It should suffice to state here that the function of the 

 external or skin layer, which is usually called an ectoderm (the 

 notion and the name have been originally created within the 

 field of the older embryology of the higher Vertebrata), had 

 only been to protect the animal and to maintain the contact 

 between the animal and its environment; while the function 

 of the entoderm had been on the whole to receive and to 

 prepare the food; the middle layer (the mesohyl or the meso- 



