204 FOUNDATIONS OF BIOLOGY 



accordingly is known as a HERMAPHRODITE. Such is the 

 condition in Hydra, where the testes appear as small swellings 

 in the ectoderm a little below the circle of tentacles; and the 

 ovary, which is usually single, is a somewhat larger projection 

 near the opposite end of the animal. Both the testis and the 

 ovary at first appear to be a heap of ectoderm cells, which in 

 one case gives rise to many sperm and in the other to a single 

 egg. The mature sperm are set free from the testis and swim 

 about in the water. Sooner or later one enters the now rup- 

 tured covering of the ovary and fuses with the egg. With the 

 conclusion of fertilization the zygote begins to divide and 

 forms an embryo, which at an early stage becomes detached 

 from the parent. Thus in Hydra there is no complicated 

 apparatus for sexual reproduction; merely now and again 

 the temporary development of the primary sex organs, 

 ovaries and testes. (Fig. 64.) 



The complex bodies of most animals, however, demand 

 more or less permanent gonads as well as means for trans- 

 ferring the gametes directly or indirectly to the exterior. 

 This is brought about by the fact that in coelomate animals 

 the gonads come to lie, not on the outside of the body, but 

 within the coelom. In the Earthworm, which also is her- 

 maphroditic, the testes and ovaries are permanent organs 

 attached to the partitions between certain somites. The 

 sexual products are set free in the coelom, where they are 

 taken up by SPERM DUCTS and OVIDUCTS and carried to the 

 outside. Although each Earthworm possesses both male 

 and female reproductive organs, two worms copulate and 

 exchange sperm which are stored in the respective seminal 

 receptacles. Later, when the eggs pass to the exterior, the 

 'foreign' sperm are shed on them. Thus cross-fertilization is 

 insured in this hermaphroditic form. In the Crayfish the 

 sexes are represented by separate individuals, and the appen- 



