MULTICELLULAR ANIMALS 195 



location of the developing zygote, animals are either egg-laying, or 

 OVIPAROUS, or give birth to living young, or viviparous. Again it 

 cannot be said that because the higher mammals are viviparous 

 this character is advanced serially throughout the scale of animal 

 complexity, for the lowest mammals lay eggs while some fishes give 

 birth to living young. 



Reproductive Organs. The sex cells are located and matured 

 in special organs known as gonads, that producing eggs being 

 termed the ovary and the sperm-producing structure the testis. 

 Testes and ovaries are temporary structures in a few animals; for 

 example, in forms like the sandworm, Nereis, the gonads appear 

 during certain seasons of the year and consist of masses of matur- 

 ing gametes. But in most animals the gonads are permanent or- 

 gans. The basic sex structures that are common to all gametic ani- 

 mals are the gonads; the accessory structures in most cases include 

 a set of tubes in the male that collect the matured sperms and lead 

 these products to the outside by means of a sperm duct. In species 

 in which fertilization is internal the male accessory structures 

 usually include some sort of intromittent organ. The accessory 

 female structures usually include tubes that conduct the ova away 

 from the ovaries and serve as avenues of exit from the body. In 

 those animals in which the eggs develop inside the body, these 

 tubes undergo more or less modification to provide a chamber or 

 UTERUS in which development takes place. In many lower ani- 

 mals the sexes are not separated, both ovaries and testes and acces- 

 sory sex organs of both sexes occurring in the same individual. Such 

 forms are known as hermaphrodites. The most common example 

 is the earthworm (Fig. 134), in which both sets of sex organs are 

 always present and functional at the same time. A copulatory 

 process, which insures the presence of sperms of another individual, 

 tends to prevent self-fertilization. 



The gonads together with the tubes and ducts that conduct the 

 sex cells may be termed primary sex organs. In addition to these, 



