138 THE EVOLUTION OF THE METAZOA 



logie) has tried to avoid this situation by attempting to derive 

 the protonephridia from the invaginated and immersed "ribs" 

 of Ctenophora. Fortunately, this interpretation did not find 

 any favourable reception among other zoologists because such 

 an evolution is completely devoid of probability. No other 

 attempt to interpret the origin of these protonephridia has 

 been known to me. We will approach this problem from an- 

 other standpoint. 



Our starting point is the fact that as early as in the Protista 

 an apparatus can be found for the excretion of the superfluous 

 water which constantly enters from the watery environment 

 into the cytoplasm, where it absorbs the dissolved products 

 of the metabolism. This is the contractile vacuole which may 

 have developed from the small intermediate cavities that occur 

 in the cytoplasm. A high level in the evolution of these vacu- 

 oles has been reached in the swimming Euciliata so that in 

 this case w^e are entitled to speak of a special excretory- 

 emunctory organelle (Fig. 23). This organelle is particularly 

 well developed in species that live in fresh water; it shows 

 a considerable lability because it can easily disappear. This 

 frequently takes place in species that live in the sea and in 

 those w^hose size has been greatly diminished. 



From this point of view we can come to the conclusion that 

 the ancestors of the Turbellaria had inherited from their Eucili- 

 ata ancestors (apart from numerous other things) an emunc- 

 tory system also as well as the middle layer which also contains 

 the body muscles. It should be remembered in this connection 

 that as early as in Euciliata this emunctory system had been 

 developed into a canal which later becomes its regular form 

 in the Eumetazoa. From the orifice of these excretory canals, 

 i.e. from the porus excretorius, the skin layer (actually quite 

 similarly to the margin of the cytostome where it has led 

 to the formation of the cytopharynx), together with its cilia, 

 has moved by way of invagination into the interior of the 

 body, and in this way the ciliated and pulsating terminal and 

 canal wall cells have been developed. In this connection we can 



