MORPHOLOGY OF ANIMALS 



through the mesonephric tubules and segmental ducts 

 (epididymis and vas deferens, Fig. 21, D, ef^ vd). 



Semination. In many aquatic animals the ova and sper- 

 matozoa escape directly into the water, and there the eggs 

 are fertilized and undergo development; it is probable 

 that in these animals the escape of ova stimulates the 

 males to eject spermatozoa so that both kinds of sex cells 

 are shed at about the same time. In such cases enormous 

 numbers of sex cells are produced and very many are 

 wasted. A slight advance over this condition is found in 

 those animals (frogs, bony fishes, etc.) in which the open- 

 ings of the male and female ducts are placed close to- 

 gether at the time of shedding of the sex cells; this is 

 known as external copulation. In other cases the spermato- 

 zoa only escape from the body, and by means of currents 

 of water they are carried into the body of the female, 

 where they fertilize the ova in situ^ as in sponges, or they 

 are carried into certain receptacles, into which the eggs 

 also are collected, as in fresh-water mussels. In other 

 animals copulatory organs exist which serve to introduce 

 spermatozoa into the sex ducts of the female, thus in- 

 creasing the chances for the fertilization of the ova; this 

 is known as internal copulation. In many cases copulation 

 occurs rarely, sometimes but once, and the spermatozoa 

 are stored in a seminal receptacle which opens into or 

 near the oviduct. Internal copulation is a necessity in all 

 land animals and in most parasites, and it also occurs in 

 many aquatic forms (flatworms, round worms, rotifers, 

 gasteropods, cephalopods, annelids, arthropods). 



Secondary Sexual Characters. In certain animals the 



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