230 COMPARATIVE ANATOMY 



the frog and the dogfish. This system is, in fact, higljly 

 characteristic of all chordate animals. 



The excretory organs or kidneys of the dogfish differ 

 considerably both in extent and in their relations to the 

 generative organs in the two sexes. They occupy the same 

 relative position as the kidneys of the frog, but are of greater 

 extent, especially in the male dogfish, and exhibit' more 

 clearly some of the primitive characteristics of the vertebrate 

 excretory system. It should be noticed that they are covered 

 ventrally by the peritoneal membrane, and therefore lie outside 

 the coelom. 



The vertebrate kidney consists essentially of a series of 

 paired coiled glandular tubules, each of which has a ciliated 

 funnel opening into the coelom. In development the pairs 

 are formed in succession from before backwards. At first 

 each pair corresponds to a body segment, but later in develop- 

 ment additional tubules are formed in connection with the 

 pre-existing pairs, and the whole organ becomes surrounded 

 with a vascular tissue and forms a solid mass. The first 

 tubules to be formed are situated far forwards in the peritoneal 

 cavity, close behind the pericardium, and they constitute 

 the primary excretory organ or pronephros. Though it is 

 present in the adults of most fishes, and is a conspicuous 

 and functional organ in the tadpole, the pronephros is a 

 very inconspicuous, functionless, and transient organ in the 

 embryo dogfish, and disappears altogether in the adult. The 

 pronephric tubules of either side of the body open into a 

 longitudinal duct, the segmental duct, which runs backwards 

 in the dorsal wall of the peritoneal cavity and opens behind 

 into the cloaca. The tubules formed next in succession to 

 the pronephros are at first independent of the segmental duct, 

 but as development proceeds they make connection with 

 and open into it, and so constitute a second section of the 

 kidney known as the mesonephros. In the frog the mesone- 

 phros is the functional kidney of the adult, and there is no 

 other section of the excretory system behind it, but in the 

 dogfish a number of excretory tubules at the posterior end 

 of the peritoneal cavity acquire separate ducts, which either 

 unite to form a common duct, or open separately into the 

 posterior section of the segmental duct, just before the latter 

 opens into the cloaca. This posterior part of the excretory 



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