192 ESSENTIALS OF ZOOLOGY 



protozoans is taken to be asexual, but according to a recently pub- 

 lished work by Sonneborn, a distinct sexuality exists in Paramecium. 

 Examples of asexual reproduction by budding and fission may be 

 found in the studies of reproduction of sponges, Hydra, Planaria, and 

 even in tunicates. 



Sexual Reproduction. — In certain of the colonial Protozoa, Volvox 

 for example, the colony may reproduce for several generations by 

 asexual division of the individual cells but sooner or later the cells 

 of the colony become specialized into conjugating individuals. In 

 some forms this goes to the extent of certain cells becoming distinct 

 gametes with male and female characteristics. In such forms it is 

 possible to see foreshadowed sexual reproduction as it is known in 

 Metazoa. 



In the simplest of Metazoa, as in sponges, there are no specially 

 organized gonads for the production of germ cells, but as a rule the 

 germ cells are produced in such organs set apart for this purpose. 

 The ovary produces mature or nearly mature ova and the testis pro- 

 duces mature spermatozoa. 



Hermaphroditism is the condition in which the same individual 

 produces both ova and spermatozoa. It occurs principally in the 

 simpler Metazoa, a few higher ones, and rarely among normal 

 vertebrates. Studies made on the reproduction of Hydra have 

 shown that the gonads are temporary, both being formed by ag- 

 gregations of formative or interstitial cells between the ectoderm 

 and endoderm. After the seasonal production of germ cells is com- 

 pleted, the gonads disappear. In flatworms and annelid worms the 

 gonads are permanent structures of the mesoderm. Both ovaries and 

 testes are present. Even in these true hermaphrodites cross-fertiliza- 

 tion is insured by copulation or union in such a way that the sper- 

 matozoa of one individual fertilize the ova of another. In certain 

 other hermaphroditic forms (as some cyclostomes) the spermatozoa 

 and ova of a particular individual are usually not mature at the 

 same time. 



Bisexual reproduction is the form of reproduction common to 

 many groups of the higher invertebrates and nearly all vertebrates. 

 Here the sexes are distinct, each with functional gonads and ac- 

 cessor}^ structures capable of producing only one kind of germ cells. 

 In some of the types of animals, individuals of the two sexes simply 

 deposit the mature germ cells in the same vicinity and at about 



