78 ANATOA/y OF AMPHIOXUS. 



pharynx, so as to enclose a tube which corresponds to that 

 portion of the future atrial cavity which lies between the 

 atriopore and the hinder end of the pharynx. Finally, 

 in Fig. 38 C, the closure of the atrial tube has advanced 

 forwards over the gill-slits almost to the anterior extremity 

 of the pharynx, still leaving, however, one or two gill-slits 

 open directly to the exterior in front. Meanwhile, the 

 floor of the atrium has increased in width, and the meta- 

 pleural folds are separated by a wider interval than before 

 (Fig. 38 C). Eventually the atrium closes up completely 

 in front, so that the gill-slits no longer open directly to 

 the exterior. 



Remembering that the atrium of Amphioxus arises as an 

 unpaired median tube (see below, IV.), while the pro- 

 nephric duct is always paired, the following are some of 

 the reasons for supposing a partial homology between the 

 two structures : — 



(a) They are both derived, either wholly (atrium), or in 

 a large measure (pronephric duct), from the ectoderm.* 

 (/3) They both receive and carry away the excretory prod- 

 ucts from the pronephric tubules ; and (7), they are 

 both, to a greater or less extent, lined by an epithelium, 

 which is itself glandular and excretory.' 



Comparisoji between the Excretory System of Ampliioxics 

 and that of the Annelids. 



Having considered the relation existing between the 

 pronephric system of Amphioxus and the corresponding 

 system in the embryonic and larval stages of the higher 

 Vertebrates, we will now pass on to a brief comparison 

 with the excretory system of the Invertebrates. 



The excretory system of a typical Annelid presents 



