142 Elementary Zoology 



nephridia are essentially glandular tubes which open on the 

 one hand into the coelom by a ciliated mouth, and on the other 

 on to the exterior. That is precisely what we find with the pro- 

 nephric tubes in the frog, and with the mesonephric tubes in 

 both frog and fowl. But while the tubes of the earthworm 

 open separately, those of the Vertebrata join to form a con- 

 tinuous duct, which, however, reaches the exterior by the 

 cloaca itself, an invagination of the epiblast of the embryo. 

 This fusion of separate nephridia does not, it is true, occur in 

 the common earthworm ; but Annelids are known in which 

 several nephridia do unite to form a continuous longitudinal 

 duct. Another apparent point of difference is that, while in the 

 earthworm each segment of the body has but a single pair of 

 nephridia, the mesonephric tubules, at any rate, are, for the most 

 part, more numerous than the segments which they occupy ; 

 here, again, there is no real difference in the matter from 

 segmented worms ; for among the Annelids more than one form 

 is known in which the nephridia show as little regard for the 

 segmentation of the body as is evinced by the mesonephros. 



