108 GENERAL PRINCIPLES OF ZOOLOGY 



such animals are called separate-sexed, diivcimis or gonochoristic, in 

 opposition to the hermaphroditic forms, in which both kinds of sexual 

 glands are contained in one and the same individual. Different degrees 

 of hermaphroditism can be distinguished; commonly testes and ovary 

 are contained in the same animal, some distance apart, as in the earth- 

 worm, in which two segments are male, while a third segment is female 

 (fig. 73). More rarely there is a union of testes and ovary into a single 

 hermaphroditic gland; land-snails have an hermaphroditic gland, which' 

 produces spermatozoa and eggs in the same follicle. 



Occurrence of Hermaphroditism. Hermaphroditism is, in general, of 

 more frequent occurrence in the lower than in the higher animals. Insects and 

 vertebrates are, almost without exception, dioecious; only among the teleosts is 

 hermaphroditism not rare. It also occurs in the myxinoids. More commonly 



FIG. 74. Lateral hermaphroditism of a gipsy moth (Ocn r ria dispar). Left female, 



right male (after Taschenberg). 



hermaphroditism occurs as an abnormality; a striking form is lateral her- 

 maphroditism, in which one half of the animal has only male, the other half only 

 female, gonads. If the males and females of a species be distinguishable by 

 their appearance, then lateral hermaphroditism is expressed in their external 

 form, since one half of the animal has the characteristic marks of the male, the 

 other half those of the female (fig. 74). Still it must be noted that, in many 

 instances where the external appearance pointed toward hermaphroditism 

 anatomical investigation has disclosed either only male or only female sexual 

 glands in a rudimentary condition (gynandromorphism). True hermaphroditism 

 (the presence of both kinds of sexual glands in the same animal) is extremely 

 rare in mammals and in man. What is described as hermaphroditism is usually 

 gynandromorphism; rarely are both kinds of gonads present in the same indi- 

 vidual, and then not in a functional condition. 



The wide distribution of hermaphroditism among the lower Metazoa has 

 led to the erroneous view that this was the primitive condition, from which the 

 gonochoristic condition has been evolved. Studies on nematodes, Crustacea and 

 possibly molluscs have shown that, on the contrary, hermaphroditism has fol- 

 lowed a dioecious condition, since with the disappearance of males, the female 

 animals may develop male sexual cells before the ovaries are mature. Con- 

 trasted to this ' protandry' 1 a ' protogyncccy' is rare. 



Genital Ducts. Very frequently the excretory apparatus furnishes 

 outlets for the sexual products. In the annelids and vertebrates portions 



