THE REPRODUCTION OF ANIMALS 



223 



and oikos, "house")- The vast majority of bisexual animals are dioecious. 

 But there are species, genera, and even families and classes of animals in 

 which the sexes are not separate. In these groups individuals possess both 

 testes and ovaries, and produce both spermatozoa and eggs. Such an 

 individual or species is said to be hermaphroditic, and the condition is 

 termed hermaphroditism. 1 Among many snails, annelid worms, flatworms, 

 and certain other organisms hermaphroditism is the normal condition. 



Fig. 15.6. A pair of earthworms in amplexus. The anterior ends of the worms are held to- 

 gether by a slime tube secreted by epidermal glands. Sperm from each worm are transferred 

 to a storage chamber (spermatheca) in the partner; then the worms separate. Later each 

 secretes a slime cocoon around a part of its body called the clitellum. This cocoon is slipped 

 off over the worm's head, the worm's own eggs and the partner's sperm being deposited in 

 the cocoon during this process. Fertilization and development occur within the cocoon in a 

 liquid that provides the necessary "aquatic" environment. (Courtesy General Biological 

 Supply House, Inc.) 



It is unknown in vertebrates except as a rare abnormality, and most 

 alleged examples in man and other mammals are not true cases where 

 testis and ovary occur in the same individual. Even in true herma- 

 phrodites cross-fertilization is often insured, as in the earthworm (Fig. 

 15.6) by arrangements of the reproductive organs such that the eggs of 

 one individual can only be fertilized by sperm from another. In other 



1 From the Greek myth of Hermaphroditus, son of Hermes and Aphrodite, who 

 was united with the nymph Salmacis to form a single androgynous (male + female) 

 person. 



