356 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the blood-vessels in their early stages (fig. 376) and this relation 

 persists through life. 



In the fishes interrenals and suprarenals are separate (fig. 377), 

 the suprarenals often being more or less metameric and in close rela- 

 tions to the blood-vessels supplying the mesonephros and those 

 farther in front. The interrenals are more compact and in the fishes 

 they lie between the excretory organs of the two sides of the body. 



In the tetrapoda the two organs are more closely associated, the 

 tissues of the two being mixed in the adults of amphibia and reptiles, 

 while in the mammals the interrenal tissue is on the outside, forming 

 the cortex of the adrenal (frequently called the suprarenal) body 

 which lies at the anterior end of the kidney. The suprarenal tissue 

 is the medulla of the adrenals. It consists of numerous phaeochrome 

 cells with small aggregates of nerve cells, and forms a series of tubules 

 through which blood supplied by the suprarenal artery circulates 

 before being carried away by the vein. In the amphibia the adrenals 

 are closely associated with the Wolffian bodies, either being attached 

 to the inner margins (urodeles), or forming yellow stripes (anura) on 

 the ventral surface. In the reptiles they are lobulated bodies near 

 the gonads. 



Both are organs of internal secretion, their product being passed 

 directly into the blood. The medullary portion in mammals forms 

 adrenalin, an activator or hormone, which by its action on the muscles 

 causes an increase in the blood pressure. Less is known concerning 

 the function of the interrenal organ, but it seems that in part it 

 destroys certain toxins, the products of metabolism, which otherwise 

 would be injurious to the organism. 



UROGENITAL ORGANS IN THE SEPARATE CLASSES 



CYCLOSTOMES. — In this class the nephrostomes are lost with the excep- 

 tion of the most anterior, while the pronephric duct becomes degenerate between 

 the anterior end and the functional part of the excretory organs. 



In the lampreys the pronephros extends over thirteen somites, but only the 

 anterior five form complete tubules, the remainder, however, join the pro- 

 nephric duct. The pronephros is best developed in the Ammocoete, 10 mm. long, 

 and in this stage the mesonephros is also developed and both are functional. 

 With increase in size there is a degeneration of the meson ephric tubules in front 

 and a formation of new ones behind, the definitive organ extending over about 

 two-fifths of the body length. Each pronephros projects into the ccelom as a 

 band supported by a fold of the peritoneal membrane. The two pronephric 

 ducts unite a little in front of the hinder end, forming a urogenital sinus into 



