EXCRETORY ORGANS. 729 



(Teleostci, Ganoidei) or be divided into two, three or more 

 apertures (Amphibia). The main part of the gland in either 

 case is formed by convolutions of the tube connected with the 

 peritoneal funnel or funnels. The peritoneal funnels of the 

 pronephros appear to be segmentally arranged. 



The pronephros is distinguished from the mesonephros by 

 developmental as well as structural features. The most im- 

 portant of the former is the fact that the glandular tubules of 

 which it is formed are always outgrowths of the segmental duct ; 

 while in the mesonephros they are always or almost always 1 

 formed independently of the duct. 



The chief structural peculiarity of the pronephros is the 

 absence from it of Malpighian bodies with the same relations as 

 those in the meso- and metanephros; unless the structures found 

 in Myxine are to be regarded as such. Functionally the place 

 of such Malpighian bodies is taken by the vascular peritoneal 

 ridge spoken of in the previous pages as the glomerulus. 



That this body is really related functionally to the pronephros appears to 

 be indicated (i) by its constant occurrence with the pronephros and its 

 position opposite the peritoneal openings of this body ; (2) by its atrophy at 

 the same time as the pronephros ; (3) by its enclosure together with the 

 pronephridian stoma in a special compartment of the body-cavity in 

 Teleostei and Ganoids, and its partial enclosure in such a compartment in 

 Amphibia. 



The pronephros atrophies more or less completely in most 

 types, though it probably persists for life in the Teleostei and 

 Ganoids, and in some members of the former group it perhaps 

 forms the sole adult organ of excretion. 



The cause of its atrophy may perhaps be related to the fact that it is 

 situated in the pericardial region of the body-cavity, the dorsal part of which 

 is aborted on the formation of a closed pericardium ; and its preservation in 

 Teleostei and Ganoids may on this view be due to the fact that in these types 

 its peritoneal funnel and its glomerulus are early isolated in a special cavity. 



Mesonephros. The mesonephros is in all instances com- 

 posed of a series of tubules (segmental tubes) which are 

 developed independently of the segmental duct. Each tubule is 



1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an 

 exception to the general rule that there is no outgrowth from the segmental or 

 metanephric duct to meet the segmental tubes. 



