258 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



excretory organs present some very complex characters 

 as we ascend the series. In the examination of these 

 organs it will be found convenient to make use of cer- 

 tain technical terms. 



The Pronephros (or head-kidney) is a small 

 glandular body, with one or more funnel-shaped 

 ciliated openings into the body cavity ; it is ordinarily 

 placed far forwards, and is provided with a duct, the 

 so-called segmental duct. Like many other renal 

 organs, it is provided with a special blood-supply in the 

 shape of a coil of vessels, or glonierulus. 



The mesonephros (Wolfftan body) consists of 

 a series of glandular tubes which open by funnel- 

 shaped openings into the body cavity, and pour their 

 secretion into the common segmental duct. 



The metanephros (kidney of Amniota) con- 

 sists of a complex of coiled tubes which open into a 

 special duct, which is derived from that of the 

 mesonephros. 



All these three organs may be developed in one 

 and the same individual, but they are not, in higher 

 forms, in active function at the same time ; the 

 metanephros is developed in the Amniota only, though 

 an indication of its existence is to be seen in Elasmo- 

 branchs. 



In the adult Cyclostoniata the mesonephros is 

 found in its simplest conditioL, for it there consists of 

 a segmental duct with tubes (Fig. 106; a, b) given off on 

 one side, and ending in a blind enlargement ; in this 

 enlargement an artery (d) breaks up into a plexus of 

 fine vessels which form the glomerulus (c), and thence 

 the blood passes into the efferent artery (e). An- 

 teriorly to this there is, in Myxine, a pronephros, 

 which disappears in the adult Petromyzon, where the 

 whole kidney is more compact. 



In the Elasmotoranchii the segmental tubes in 

 the hinder part of the mesonephros unite with one 



