EXCRETORY ORGANS. 577 



Cyclostomata. The development of the excretory system amongst 

 the Cyclostomata has only been studied in Petromyzon (Muller, 

 Fiirbringer, 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 grovi^s backwards by a continuous process of differen- 

 tiation 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 arrange 

 ment, 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 pronephros 

 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 functional part of the 

 excretory system. 



A mesonephrosis formed (Fiirbringer) relatively late in larval life, 

 as a segmentally arranged series of solid cords, derived from the peri- 

 toneal 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 couipleted when 

 the Ammocoete has reached 180 mm. in length. Further changes 

 take place in connection with the excretory system on the con- 

 version of the Ammocoete into the adult. 



The segmental ducts in the adult fall into a common urinogenital 

 cloaca, which opens on a papilla behind the anus. This cloaca also 

 communicates by two apertures (abdominal pores) with the body 

 cavity. The generative products are carried into the cloaca by these 

 pores ; so that their transportation outwards is not performed by 

 any part of the primitive urinary system. The urinogenital cloaca 

 is formed by the separation of the portion of the primitive cloaca 

 containing the openings of the segmental ducts from that connected 

 with the alimentary tract. 



The mesonephros of the Ammocoete undergoes at the meta- 



B. K II. 37 



