NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 47 



The reproductive elements of the anneHds are hberated in various 

 ways. In some of the Archiannelida and Polychaeta there is no 

 anatomical provision for the discharge of the sex products from the 

 coelomic sacs, and in such cases the gametes escape by a rupture of 

 the body wall or by fission of the rear part of the body. With certain 

 polychaetes having closed nephridia, a funnel-shaped structure is 

 developed in the genital somites on the anterior surface of the septum, 

 which at maturity opens into the canal of the nephridium, and serves 

 as an outlet for the gametes ; but again in others the funnel, though 

 present, is a mere "ciliated organ" of the coelomic peritoneum, not 

 known to acquire an opening. Special genital ducts with an internal 

 funnel and an external aperture are present in only a few Polychaeta, 

 as in some of the Capitellidae, but they are characteristic features of 

 the genital segments of Oligochaeta and Hirudinea. In most of the 

 Polychaeta the nephridial' funnels serve for the discharge of the 

 gametes. 



The relationship of the various types of annelid excretory organs 

 and genital ducts to one another is difficult to understand. According 

 to the well-known theory of Goodrich (1898-1900), nephridia and 

 genital ducts, or coelomoducts, originally formed two separate series 

 of segmental organs, and are still retained as such in Oligochaeta, 

 Hirudinea, and certain Capitellidae. In the majority of the Poly- 

 chaeta, however, Goodrich claimed, the genital funnel has lost its own 

 duct and its funnel has united with the mouth of the nephridium, 

 intermediate stages being suggested in some forms where there is a 

 partial fusion between the funnel and the nephrostome. 



The study of the development of the open nephridia has given rise 

 to much difference of opinion as to the origin of the nephridial 

 rudiments. The earlier investigators, such as Hatschek and Vejdov- 

 sky, regarded the nephridial funnels and canals as mesodermal 

 structures, but Whitman (1886) claimed that the nephridia of the 

 leech Clepsine are entirely of ectodermal origin. Wilson (1889), in 

 his work on the development of Lumhricus, described the nephridial 

 canals as being apparently ectodermal structures, developed from 

 continuous rudiments formed from the second and third rows of 

 ectodermal cells of the germ band, though he admitted they might 

 be mesodermal ; the funnels, however, he said are derived separately 

 from the anterior walls of the coelomic septa. Staff (1910) asserted 

 also that the nephridial canals are ectodermal products in Criodrilus, 

 but are formed from only the second row of cells in the germ band ; 

 and Tannreuther (1915) claimed the nephridia of Bdellodrilus have 

 the same origin, though he did not follow their complete development. 



