700 CYCLOSTOMATA. 



Cyclostomata. The development of the excretory system 

 amongst the Cyclostomata has only been studied in Petromyzon 

 (Mu'ller, FUrbringer, and Scott). 



The first part of the system developed is the segmental duct. 

 It appears in the embryo of about 14 days (Scott) as a solid 

 cord of cells, differentiated from the somatic mesoblast near the 

 dorsal end of the body cavity. This cord is at first placed 

 immediately below the epiblast, and grows backwards by a 

 continuous process of differentiation of fresh mesoblast cells. It 

 soon acquires a lumen, and joins the cloacal section of the 

 alimentary tract before the close of foetal life. Before this 

 communication is established, the front end of the duct sends a 

 process towards the body cavity, the blind end of which acquires 

 a ciliated opening into the latter. A series of about four or five 

 successively formed outgrowths from the duct, one behind the 

 other, give rise to as many ciliated funnels opening into the body 

 cavity, and each communicating by a more or less elongated 

 tube with the segmental duct. These funnels, which have a 

 metameric arrangement, constitute the pronephros, the whole 

 of which is situated in the pericardial region of the body 

 cavity. 



On the inner side of the peritoneal openings of each pro- 

 nephros there is formed a vascular glomerulus, projecting into 

 the body cavity, and covered by peritoneal epithelium. For a 

 considerable period the pronephros constitutes the sole func- 

 tional part of the excretory system. 



A mesonephros is formed (FUrbringer) relatively late in 

 larval life, as a segmentally arranged series of solid cords, 

 derived from the peritoneal epithelium. These cords constitute 

 the rudiments of the segmental tubes. They are present for a 

 considerable portion of the body cavity, extending backwards 

 from a point shortly behind the pronephros. They soon separate 

 from the peritoneal epithelium, become hollowed out into canals, 

 and join the segmental duct. At their blind extremity (that 

 originally connected with the peritoneal epithelium) a Malpighian 

 body is formed. 



The pronephros is only a provisional excretory organ, the 

 atrophy of which commences during larval life, and is nearly 

 completed when the Ammoccete has reached 180 mm. in length. 



