292 THE EVOLUTION OF THE METAZOA 



and it is unequal. In this species the essential difference 

 between the two sexes is not morphological, but rather physi- 

 ological (biochemical). While its true microconjugants (any 

 macroconjugants) are still able to conjugate mutually, the 

 microconjugants when they meet are unable to do the same. 



Sexual differentiation has pursued another direction in the 

 species Metopus sigmoides (according to Noland, 1927). The 

 two sexes do not differ morphologically, yet their destinies 

 are different. One of the conjugants loses its individuality 

 due to conjugation, and it is destroyed. Both these elements, 

 the sexual dimorphism and the unequal destiny of the two 

 conjugants can also be observed in the sessile Peritricha. The 

 macroconjugant remains sessile; it functions mainly as a 

 female even if it shows hermaphroditic inclinations because 

 it is only its gynokaryon which becomes fertilized and which 

 continues to develop. Its androgamete passes over into the 

 free and swimming microconjugant which has taken over the 

 irole of a male ; it produces one functioning androgamete only 

 w^hile all the rest get lost and they are resorbed by the luckier 

 macroconjugant. This situation reminds us to some extent 

 of some fish species that inhabit the deep sea whose dwarfed 

 males live as parasites on their larger females. Tw^o important 

 differences must naturally not be forgotten in this connection: 

 these fish are gonochoristic animals and their males are not 

 resorbed. 



Instead of speaking about the macro- and micro-conjugants 

 we could also speak of the gyno- and andro-conjugants, 

 parallel to the females and males of the gonochoristic Eume- 

 tazoa; yet these conjugants of Infusoria are hermaphrodites. 

 The process of conjugation (as a secondary sexual process) 

 becomes so one-sided that during the same process the andro- 

 <:onjugant loses his life. This, however, cannot be observed 

 among the Eumetazoa, but we can also mention the Rotato- 

 ria w^hose males can become strongly degenerated, or rather 

 they can evolve one-sidedly and in this way retrogress so that 

 finally they do not even appear! The females, however, which 



