MORPHOLOGY OF ANIMALS 



animals without a true coelom the sex cells arise in tubes 

 or glands the cavities of which may perhaps represent 

 the coelom (Fig. 5, B). In flatworms the gonads, espe- 

 cially the testes, occur in considerable numbers in a single 

 individual (Fig. 11, B). In round worms they are limited 

 to one or two elongated tubes ; in rotifers, mollusks, mol- 

 luscoids, and echinoderms they are confined to one or at 

 most a few sex glands, while in segmented animals they 

 are found in primitive forms in every body somite, though 

 with advancing organization they become limited to a 

 few somites or even to one (Fig. 12, A). In most animals 

 above the coelenterates some form of duct (gonoduct) 

 exists for carrying the sex cells to the exterior; among the 

 flatworms, roundworms, and rotifers these gonoducts are 

 never the protonephridia, though they may possibly repre- 

 sent the coelom of higher animals. In higher forms the 

 gonoducts are usually metanephridia, or modified excre- 

 tory ducts. 



Gonads of Vertebrates. In vertebrates the sexes are sepa- 

 rate. A single pair of ovaries or testes is located in the dorsal 

 part of the body-cavity near the kidney ( Fig. 2 1 , ^ ) , but in 

 almost all mammals the testes descend from this position 

 and come to lie outside the abdominal cavity in the scro- 

 tum^ where a lower temperature favors the development 

 of spermatozoa. The ovaries remain in their original po- 

 sition and when the ova in them are ripe they break out 

 into the body-cavity and are then carried to the exterior 

 through the oviducts (Fig. 21, od). Spermatozoa of ver- 

 tebrates never escape into the body-cavity but pass out 



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