Evolution of Animals 469 



We may profitably pursue the history of the reproductive 

 organs from the Nemertinea. In genera Hke Tetrastemma 

 and Geonemertes> that include species which are mainly fresh- 

 water or terrestrial, the species may be hermaphrodite, but 

 the great majority of the Nemertinea are dioecious. Amphioxus 

 is dioecious, but, while the majority of the Cyclostomata are 

 so, it has been shown evidently beyond doubt that Myxine 

 is protandrously hermaphrodite. Much weight need not be 

 given to this, though there is no sound reason why such may 

 not be a lingering ancestral inheritance. The Urodela are 

 entirely dioecious. 



In Nemertinea the reproductive organs are simple sacs 

 which lie at intervals alongside, and on either side of, the 

 alimentary canal, or between the coeca of it when such are 

 formed. They arise as swellings of cells in the body cavity, 

 which by multiplication form a lining membrane and an en- 

 closed packet of eggs and sperms. When ripe each sac becomes 

 applied to and forms a pore in the body-wall, slightly above 

 the line of each lateral nerve cord. The genital pores are 

 therefore temporary openings, extending as a double row 

 from the anterior-mid to the hind region of the body. 



In Amphioxus the sacs occupy a similar position along the 

 animal as paired, rarely unpaired, organs. Each consists of 

 an investing epithelium enclosing the eggs or sperms. But 

 when each sac is ripe, instead of opening outwardly by a sep- 

 arate pore as in the last, the contents are shed internally into 

 the atrium and escape by its pore. 



In the Cyclostomata the sacs appear as unpaired organs 

 "extending^nearly the whole length of the coelom" (139: 402). 

 In structure they agree with both above. When ripe their 

 contents are shed into the body cavity and from there are 

 carried through a pair of "genital pores" into the common 

 urino-genital sinus, and thence to the exterior. 

 iQIn the Apoda a marked advance on the above generally 

 similar condition is observed. The sacs — egg or sperm — are 

 disposed in lines along the middle part of the body. They 

 are united by a common tube in the case of the testis, which 

 in turn communicates by transverse tubes with another longi- 

 tudinally placed tube, that ultimately sheds its contents through 

 the urinary tubules and the urino-genital duct to the exterior. 



In the Urodela a distinctly segmental disposition is trace- 

 able in the ovaries and also in the oviducts, but the testes are 

 reduced and condensed into an oval mass from which a few 

 (2-4) efferent canals convey the products as in Apoda. 



