CHAPTER IX 



Sexual phenomena in multicellular animals Structure and life history of 

 Hydra and Obelia Alternation of generations The ccelomate type of 

 structure Secondary sexual characters The evolution of sex. 



IN multicellular animals or Metazoa, as in multicellular plants, 

 a sharp distinction can usually be drawn between the somatic cells 

 which build up the various tissues and are concerned with the 

 life of the individual, and the germ cells or gametes which are 

 concerned with the propagation of the race and which alone (in 

 most cases) have the power of separating from the parent soma 

 or body and giving rise to new individuals. 



In nearly all the Metazoa the gametes are sexually differen- 

 tiated into relatively large, passive ova and much more minute, 

 active spermatozoa which swim about by means of flagella. 

 The actual gametes arise by subdivision of undifferentiated 

 primordial germ cells. In the sponges, whose organization has 

 not advanced very much beyond that of complex colonies of 

 Protozoa, the primordial germ cells are merely wandering 

 amo3boid cells, resembling the white blood corpuscles of verte- 

 brates. Some of these round themselves off and give rise to 

 more or less spherical ova, others divide into spermatozoa, and 

 probably the entire sponge itself is in most cases either male or 

 female, producing one kind of gamete only. In the sponge the 

 germ cells are not localized in definite organs but scattered 

 singly or in groups throughout the gelatinous ground-substance 

 of which the body is largely composed. 



In the great majority of Metazoa, on the other hand, the 

 germ cells are segregated in well-defined organs termed gonads. 

 As a rule each gonad produces only ova, when it is known as an 

 ovary ; or spermatozoa, when it is known as a spermary or testis ; 

 only occasionally does it produce both, as in the case of the ovo- 

 testis of the snail. The gonads may accordingly be spoken of as 

 female, male, or hermaphrodite as the case may be, and the same 

 terms are also applied to the animals themselves, a male or a 



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