502 EDWIN S. GOODRICH. 



Craniata. — Owing mainly to the recent researches of van 

 Wijhe (99), Riickert (89 and 90), Seraon (95), and others, it 

 is now generally concluded that the Crauiates originally pos- 

 sessed a metameric series of pronephric tubules, or peritoneal 

 funnels, opening independently to the exterior. In existing 

 forms the funnels, which grow out from the coelomic follicles, 

 sometimes reach the epiblast as solid rods (fig. 19) ; they then 

 fuse with each other to a common duct which opens into the 

 cloaca (fig. 31), constituting what is known as the pronephros 

 and its pronephric duct. In his excellent paper on the de- 

 velopment of the renal system of Ichthyophis, speaking with 

 regard to E. Meyer's theory, Semon says : "Die segmentalen 

 Ausfiihrgange der segmentalen GenitalfoUikel oder Ursegmente 

 iibernehmen neben ihrer urspriinglichen audi noch exkre- 

 torische Funktion, sie werden zu Vornierkanalchen " (95). 



To conclude our general survey it may be said : that the 

 coelom can be traced from its smallest beginning as a cavity or 

 cavities in which are developed the gonad-cells : it grows 

 gradually in size and importance until it becomes the body 

 cavity in which the viscera rest ; that the genital ducts (with 

 a few possible exceptions due to secondary modifications) are 

 homologous throughout the Coelomata ; that the nephridia, 

 which have often been confused with these ducts, can always, 

 when they occur, be distinguished from them ; and finally, that 

 the ccelom may secondarily acquire a renal function, in conse- 

 quence of which the peritoneal funnels supersede the nephridia 

 proper as excretory ducts. 



List of Wokks referred to. 



1. Allen, E. J.— "Nephridia and Body-cavity of some Decapod Crustacea," 



' Quart. Journ. Micr. Sci.,' N. S., vol. xxxiv, 1892-93. 



2. Bateson, W. — "The Early Stages in the Development of Balano- 



glossus Kowalevski," 'Quart. Journ. Micr. Sci.,' N. S., vol. xxiv, 

 1884. "The Later Stages," &c., id., vol. xxv, suppl. 1885, and vol. 

 xxvi, 1886. 



3. Bateson, W. — " The Ancestry of the Chordata," ' Quart. Journ. Micr, 



Sci.,' vol. xxvi, 188G. 



