GENERAL ORGANOLOGY 



109 



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

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

 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 coclom. A urogen- 

 ital system occurs only in animals in 

 which the germinal epithelium arises 

 from the epithelium of the coelom, 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 deferentia 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 caUed the 

 ductus ejaculalorius; it may be evaginated 

 or project permanently beyond the sur- 

 face of the body as a penis or cirms. 

 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. 



p\e — ■ 



Fig. 75. — Vortex virdis (after 

 Schultze and von Graff): b, brain 

 with eyes; ftc, bursa copulatri.x; d, 

 digestive tract; g, genital pore; 0, 

 ovary with oviduct; pli, pharynx; 

 pe, penis; r, receptaculum seminis; 

 /, testis with vas deferens; k, uterus; 

 va, vagina; I'i, vitellariuin; vs, vesi- 

 culum seininaUs. 



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 in 

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

 arthropoda, fig. 35, e). In some cases a definite part of the ovary produces 

 these 'yollt cells,' a condition that explains the fact that in many animals (Pla- 



