294 THE EVOLUTION OF THE METAZOA 



free; the polykaryonic state has also been preserved. The 

 rejuvenating sexual process, a warrant of continuity, has 

 remaired connected with the gametes. The materral hody— 

 inasmuch as it is not saved by an asexual reproduction — is 

 exposed to an unavoidable ageing which ends every time with 

 the death of the individual. Yet this has also made possible, 

 as it has been frequently emphasized, the progressive evolution 

 of the transitory sexual phase which has finally led to the 

 formation of important and extremely differentiated species. 



There are still other paths that were pursued in the evolution 

 of complex species, i.e. such that lead by way of numerous 

 biological units (in this case of genuine, primary cells), and 

 thus of polycellular beings (Spongiae and frequently also 

 Metaphyta). In many respects they have begun to resemble 

 Eumetazoa, especially as regards their sexual phase and the 

 ontogeny, a fact which we will later return to discuss in more 

 detail. Still these forms have remained far behind the Eume- 

 tazoa, a fact that can be observed above all in the level \\'hich 

 has been reached in their organizations, and in the uniformity, 

 or in the wholeness, of their individuals. 



The instance of the Ophryoscolecida to which Dogiel has 

 called our attention shows that the nuclei of Infusoria had 

 in reality earlier been independent cells, and in certain 

 cases even true gametes. They had adapted themselves to a 

 life in the stomach of the Ruminantia; they have thus become 

 strongly specialized and they can in no way be considered 

 as primitive forms. These remarkable minute animals have 

 developed special armour-like formations which cover the 

 whole surface of their body, with the exception of its terminal 

 oral area and which protects the animal against the effects of 

 gastric juices. The "bridging" during conjugation by way of 

 cytoplasm has thus become impossible, while at the same 

 time conjugation itself has been preserved. The contact 

 of the two conjugants has been modified ; the two indivi- 

 duals, which are usually dimorphous, touch each other \\-ith 

 their terminal ends w^hich are not covered with the armour- 



