INTERNAL ANATOMY. 65 



tion to the primary gill-clefts, which are much more 

 numerous than the myotomes in the region of the body 

 in which they occur, their arrangement is more strictly 

 defined as branchiomeric. In the larva, however, the pri- 

 mary gill-slits correspond numerically with the myotomes 

 or muscle-segments of the pharyngeal region, only sec- 

 ondarily becoming more numerous. The branchiomeric 

 arrangement of the excretory tubules of Amphioxus need 

 not, therefore, prejudice their claim to be regarded as 

 segmcntal structures. 



If, now, we attempt to compare the nephridial system 

 of Amphioxus with the kidney of the higher types, we 

 shall find that here also, as in so many other instances, 

 the permanent state of things in the former becomes a 

 characteristic feature of the embryo in the latter. 



As is well known, the kidney of the higher Vertebrates 

 comprises a mass of convoluted tubules, the uriniferous 

 tubules, imbedded in a matrix of fibrous connective tissue, 

 and enclosed within a common sheath, and so producing 

 collectively a compact organ which we call the kidney. 



If, neglecting the highly elaborate structure presented 

 by the kidney of Birds and Mammals, we take, as a typi- 

 cal example of a primitive Vertebrate renal organ, that of 

 a tailed Amphibian, we find after a superficial examina- 

 tion the following characteristic features. In the newt, 

 for instance, the surface of the elongated kidney is studded 

 with numerous small apertures. These are surrounded by 

 vibratile cilia, and lead directly from the body-cavity into 

 the convoluted renal tubules. They are, therefore, the 

 ccelomic openings or funnels of the latter, and are known 

 as nephrostomes. Close to the nephrostome a short diver- 

 ticulum of the tubule leads to a capsule which encloses a 

 glomcrulus. After a winding course in the substance of 



