GENERAL ORGANOLOGY 



of the nephridial system, either exclusively or in addition to their excretory 

 function, become genital ducts. Hence we speak of a urogenital system. 

 This connection of genital and excretory organs has a double cause. 

 Physiologically important is the fact that eggs and spermatozoa behave 

 like excreta; substances which are no longer needed by the individual, but 

 must reach the exterior in order to be- 

 come of use. The morphological cause 

 is the relation to the coelom. A urogen- 

 ital system occurs only in animals in 

 which the germinal epithelium arises 

 from the epithelium of the ccelom, and 

 in which the kidneys or their rudiments 

 are in connection with the body cavity 

 and thus form the natural outlet for its 

 products. Whether the accessory sexual 

 parts are portions of the excretory organs 

 or are independent structures, they have 

 in the animal series a definite arrange- 

 ment adapted to their function (figs. 73 

 and 75). Canals lead from the gonads 

 to the exterior, oviducts in the female, 

 vasa defercntia in the male (and the 

 hermaphroditic duct from the hermaph- 

 roditic gland). 



Accessory Sexual Apparatus. The 

 terminal portion of the vas deferens is 

 often very muscular and is called the 

 ductus ejaculatorius; it may be evaginated 

 or project permanently beyond the sur- 

 face of the body as a penis or cirrus. 

 The terminal portion of the oviduct is 

 often widened so that two portions may 

 be distinguished, the uterus, which har- 

 bors 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. 



Occasionally, in the animal kingdom, a part of the eggs degenerate and are 

 used for the nourishment of the others. This degeneration may take place 

 the uterus (Salamandra), in the egg cocoons (annelids), or in the ovary ( 

 arthropoda, fig. 35, <). In some cases a definite part of the ovary produ 

 these 'yolk cells,' a condition that explains the fact that in many animal.-, (Fla- 



P \e \- 



FiG. 75. Vortex rinlis (after 

 Schultze and von GratT): b, brain 

 with eyes; be, bursa copulatrix; </, 

 digestive tract; g, genital pore; o, 

 ovary with oviduct; pit, pharynx; 

 pe, penis; r, receptaculum seminis; 

 t, testis with vas deferens; it, uterus; 

 i'a, vagina; i'i, vitellarium; r.v, vesi- 

 culum seminalis. 



