120 



GENERAL PRINCIPLES OF ZOOLOGY. 



Genital Ducts. Very frequently in the animal kingdom the 

 excretory apparatus furnishes outlets for the sexual products. In 

 the annelids and in the vertebrates portions 

 of the nephridial system, either exclusively 

 or in addition to their excretory function, 

 become accessory sexual organs. Hence we 

 speak of a urogenital system. This remark- 

 able connexion of genital and excretory 

 organs has a double cause, a physiological and 

 an anatomical. Physiologically important is 

 the fact that eggs and spermatozoa behave 

 like excreta; being substances which are no 

 longer destined for the use of the individual, 

 but must reach the exterior in order ta 

 become efficient. The morphological cause 

 is the relation to the co3lom. A iirogenital 

 system is formed only in animals in which the 

 germinal epithelium arises from the epithe- 

 lium of the ccelom, and in which the kidneys 



FIG. 73. Sexual appara- ,-\ j- j j_i 



tus of Vortex vtridis. or their rudiments stand permanently in 



glands - 



connexion with the body cavity and thus 

 form the natural outlet for its products. 

 Whether the accessory sexual parts are por- 

 tions of the excrefcor J organs or are inde- 

 pendent structures, they have in the animal 

 series a definite arrangement adapted to their function (fig. 73). 

 Canals lead from the sexual glands to the exterior, the oviducts in 

 the female, the vasa defer entia in the male (and the herma- 

 phroditic duct from the hermaphroditic gland). 



Accessory Sexual Apparatus. The terminal portion of the vas 

 deferens is often very muscular and is called the ductus ejacula- 

 torius; it may be evaginated as a penis or cirrus, or project 

 permanently beyond the surface of the body. The terminal 

 portion of the oviduct is often widened so that two portions may 

 be distinguished, the uterus, which harbors the eggs during their 

 development, and the vagina, which serves for copulation. In 

 addition there may occur in both sexes other accessory glands of 

 the most diverse character. Oviduct and vas deferens may be 

 provided with sac-like evaginations which serve for the reception 

 of the sperm. In the female these are called receptacula seminis, 

 in the male vesiculce seminales; the former give lodgment to sperm 

 which enters the female sexual passages during coition, the latter 



