IV. VERTEBRATA. 487 



in close proximity to the nephrostome. This may consist of several net 

 works or they may be united to a single large glomus, situated in a special 

 capsule of the coelom. 



The excretory system of the vertebrates is farther distinguished from 

 that of the annelids in that (i) the nephridial tubules do not open separately 

 to the exterior but by a common canal on either side; (2) the segmental 

 arrangement is largely obliterated by the development of secondary, 

 tertiary, etc., tubules which produce a compact glandular body. The 

 accurate explanation of these relations requires a knowledge of the 

 different kinds of excretory organs in the vertebrates, for which we are 

 indebted to comparative embryology. Of these there are three kinds: 

 (i) pronephros or head kidney, (2) mesonephros or Wolffian body, (3) 

 metanephros, or true kidney, the relative positions of which along the verte- 

 bral column are indicated by their Greek names. 



The pronephros is greatly reduced in all vertebrates with the possible 

 exception of some cyclostomes; it is functional but rarely in the larva (am- 

 phibians) and is confined to the anterior segments. Whether it formerly 

 extended farther back, as has been suggested, is cjuestionaljle. Its vas- 

 cular" system is a glomus, belonging to the ccelom and its short (pro- 

 nephridial) tubules unite distally to form the pronephi-ic duct, which grows 

 back from its point of origin to the cloaca, into which it opens. In the 

 elasmobranchs a so-called Muller's duct splits from the pronephric duct, 

 retaining its connection with the pronephros. In the other vertebrates 

 the Miillerian duct develops from the pronephros as a canal growing 

 from in front backwards, and opening in front into the ccelom by large 

 ciliated funnel, the ostium tubce ahdominale, behind into the cloaca. Hence 

 Muller's duct belongs to the pronephric system. It develops in the female 

 into the oviduct and uterus, degenerates in the male. Whether the pro- 

 nephros is an individual excretory organ, or a part of the same system as 

 the meso- and metanephroi is disputed. 



The mesonephros is a considerable organ in all vertebrates, function- 

 ing in amniotes only in fcetal life, and later being replaced functionally 

 by the metanephros and degenerating to a few inconsiderable remains 

 in the male sexual organs. Its tubules bear Malpighian bodies and unite 

 secondarily with the pronephric duct, which now is called the Woljfiati 

 duct. The organ has a compact character because additional tubules are 

 added to those first formed, these being more numerous farther back. 

 These later tubules do not empty directly into the duct but into the primary 

 tubules, the terminal section of which thus becomes a collecting tubule. 

 In the ca?cilians there is a modification in that the secondary canals 

 connect with branches from outpushings from the Wolffian duct, this 



