MORPHOLOGY OF ANIMALS 



velopment without fertilization occurs in larval forms 

 which have not completed their development it is known 

 as paedogenesis. In many animals the sexes are separate, — 

 that is, ova and spermatozoa are produced by dif- 

 ferent individuals, males and females, and the species is 

 dioecious; in some cases, however, both kinds of sex cells 

 are produced by the same individual, which is then said 

 to be hermaphrodite^ and the species is monoecious. Sepa- 

 rate sexes were probably derived from hermaphrodites by 

 the suppression of female organs in the male and of male 

 organs in the females; in fact each sex has rudiments of 

 the organs of the opposite sex. 



Ovaries and Testes. The essential reproductive organs 

 are the gonads, or the glands which produce ova and sper- 

 matozoa, namely the ovaries and the testes. In sponges 

 the reproductive cells are widely scattered through the 

 mesoderm so that in these animals ovaries and testes can- 

 not be said to exist. In the lowest cnidarians (Hydrozoa) 

 the sex cells are at first widely scattered in the ecto- 

 dermal epithelium, but they actively migrate to certain 

 portions of the hydroid stem where reproductive buds are 

 being formed, and, aggregating there, form gonads. A 

 similar migration of sex cells into the gonads has been 

 described in several vertebrates. In all higher animals 

 definite gonads are present. 



No genital ducts are present in the coelenterates and 

 none are needed, since the sex cells can escape directly into 

 the water in which they live. In animals above the coelen- 

 terates the sex cells are mesodermal in origin, and in most 

 cases form a part of the epithelium lining the coelom. In 



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