i86 



COMPARATIVE ANATOMY 



the ventral vessel and backwards in the dorsal vessel. This 

 is characteristic of all vertebrates, and is exactly the reverse of 

 what happens in worms and Arthropods. 



The excretory organs of Amphioxus are remarkable because 

 of their close resemblance to the nephridia of many polychaste 

 worms, and because they differ in some important respects 

 from the excretory organs of fishes and higher vertebrata. 

 They are undoubtedly nephridia, but unlike those of the earth- 



Fig. 46 



i. Three nephridia with the neighbouring portion of the pharyngeal wall, after Boveriand 

 Goodrich, at.o, atriai opening of a nephridium ; cte, dorsal coelom passing into a 

 coelomic canal in a primary gill-bar : /, a nephridium with short dorsal branches 

 bearing solenocytes ; sy, cross-bar or synapticulum ; P,P, primary gill-bars; T,T, 

 tongue-bars. 2. A portion of an excretory canal bearing a group of solenocytes, 

 semi-diagrammatic, after Goodrich, ex, excretory canal : , nucleus; t, tube with 

 flagellum. 



worm, they have no ciliated funnels opening into the ccelomic 

 cavity, and though they are serially arranged, they do not 

 correspond with the muscle segments of the body, but with 

 the gill slits, which, as has been explained above, are much 

 more numerous than the muscle segments of the pharyngeal 

 region. There are about a hundred excretory tubes or 



