THE OH^TOGNATHA, OR PRIMITIVE MOLLUSCA. 377 



The origiu of tlie genital cells side by side from the walls 

 of the same cavity, and tlie subsequent division of tliis 

 cavity into anterior and posterior parts by a septum of quite 

 peculiar growth, may be given as reasons against the view 

 that the so-called trunk and tail cavities of Chsetognatha are 

 indications of metamerism. 



Agaiu, if the trunk and tail cavities were homomeric, we 

 should expect to find their dncts also homomeric. This is 

 certainly not the case, for the efferent genital ducts of the 

 Chastognatha are essentially different both in structure and 



Ovd. 



Ov. 



Text-fig. 9.— Ova and follicle cells. 

 A. Clisetoguath (after Hertwig). b. Lamellibranch (after Pelseneer). 



in development. The sperm-ducts being ciliated, and the 

 oviducts being devoid of ciliation, was clearly established by 

 Hertwig in 1880, and should have made later writers hesitate 

 before adopting a theory of metameric segmentation ; but 

 quite recently the embryological studies of Doncaster have 

 indicated that the two ducts are derived from different germ- 

 layers — the sperm duct from epiblast, and the oviduct from 

 mesoblast. 



We suggest that the oviduct of Chsetognatha is merely a 

 gouoccel, partly lined by the follicular epithelium covering 

 the egg-cells, which has acquired an opeuiug of its own to 

 the exterior (text-fig. 9). 



