184 PRINCIPLES OF ANIMAL BIOLOGY 



Hermaphroditism. — Most animals possess either male or female 

 organs of reproduction but not both. This is the condition in the 

 great majority of species of the metazoa. Species which have sepa- 

 rate sexes are said to be dioecious (living in two houses) while those 

 species whose individuals produce both eggs and sperms are called 

 monoecious (living in one house). Individuals with both male and female 

 organs are known as hermaphrodites and they exhibit the phenomenon 

 called hermaphroditism. True hermaphroditism is rare among verte- 

 brates but traces of it are found in certain cyclostomes and commonly 

 among Amphibia. The invertebrates present many cases of true her- 

 maphroditism. Hydra viridissima and H. vulgaris are common labora- 

 tory examples. Most species of flatworms including the turbellarians, 

 cestodes or tapeworms, and trematodes or jElukes, are hermaphroditic. 

 A few species of nematodes or roundworms and most snails are also 

 hermaphroditic. In many species the spermatozoa are produced first 

 and later the ova, but in some species this condition is reversed. By 

 developing the sexual products at different times cross-fertilization 

 is assured. In the earthworm, eggs and sperms are produced in the same 



Fig. 142. — Earthworms, Helodrilus fcetidus (Savigny), copulating. (After Foot.) 



individual and at the same time. Cross-fertilization is assured in this 

 case by the arrangement of the generative organs and by the method of 

 mating. 



Mating in the earthworms may occur under the surface of the soil, 

 or in a compost heap as in the case of Helodrilus fcetidus (the manure 

 worm), or above the soil as in Lumbricus terrestris, the large worm gen- 

 erally used for laboratory dissection. In these species the bodies of the 

 two worms are closely applied bj^ their ventral surfaces, the heads pointing 

 in opposite directions and the thickened band or clitellum of each worm 

 approximately opposite segments 7 to 12 of the other worm (Fig. 142). 

 In this position each worm secretes a slime tube which encircles its body 

 from about segment 8 to segment 33 or even as far back as 39. In some 

 instances, at least, the slime tubes of the two worms seem to be indepen- 

 dent of each other. In the region of the clitellum of each worm thickened 

 rings of the slime tube encircle both worms and l)indthem closely together. 

 The spprms are discharged from the spermaducal pores into the space 

 bounded by the slime tube and the })od3' of the worm. Here the sperms 

 form into irregular masses or spermatophores which are carried backward 



