180 



PRINCIPLES OF ANIMAL BIOLOGY 



fertilization is conceivable. This expectation is not usually realized, 

 however. Relatively few animals are hermaphroditic, among them being 

 some sponges, Hydra and a few animals similar to it, worms, and snails. 

 Though hermaphroditism is often described in various kinds of vertebrate 

 animals, the condition so named is usually merely the existence of egglike 

 and spermlike cells in the same gonad. Since usually only one, if either, 



o—. 



Fig. 149. — Genital organs of a hermaphi-odite animal, a common land snail Polygyra 

 albolabris (Say). Note that some of the organs are characteristic of a male, others of a 

 female. 1, atrium; 2, penis; 3, prepuce; 4, vagina; 5, spermatheca; 6, vas deferens; 7, free 

 oviduct; 8, uterus; 9, spermatic duct; 10, talon; 11, hermaphroditic duct; 12, hcrmaphioditic 

 gland; 13, penis retractor; 14, albumen gland. 



of these kinds ever reaches maturity and since appropriate ducts for 

 leading off both kinds of cells are not often present, such animals are not 

 functional hermaphi-odites at all. The reproductive system of a really 

 hermaphroditic animal, a snail, is shown in Fig. 149. 



Most hermaphroditic animals have some way of avoiding self-fertiliza- 

 tion. In some of them, though l)oth kinds of germ cells are produced, 

 eggs predominate in some individuals, spermatozoa in others, and mere 

 chance favors cross-fertilization. In other animals, the two kinds of 



