354 THE ANIMAL KINGDOM. 



stitute the absorbent or lacteal system. The larger vessels have well 

 defined and independent walls, and in Aves and Mammalia valves like the 

 veins. A large lymphatic vessel or reservoir, the ductus thoracicus, lies at 

 the back of the thorax in Aves and Mammalia. It communicates with the 

 two venae cavae superiores in the former, with the left one only in Mam- 

 malia. There is a lymph sinus in the same position in Amphibia and 

 Reptilia. This sinus, like the duct, receives lymph from the hinder extremi- 

 ties, body walls and viscera. A lymphatic tissue, known as adenoid or 

 reticular tissue, consisting of a net-work of cells, which bud and form lymph 

 or white corpuscles, is greatly developed in the submucous coat of the 

 intestine, and as masses or lymphatic glands here and there in the course of 

 the lacteal and lymphatic vessels of higher Vertebrata. 



The supra-renal bodies usually found in close apposition with the 

 kidneys or genitalia, appear to consist of a portion (the medulla in higher 

 forms) derived from the sympathetic nervous ganglia : and another portion, 

 the cortex, derived from mesoblast, and in higher forms in immediate con- 

 nection with the vena cava inferior and cardinal veins. The two parts 

 remain separate in Elasmobranchii the mesoblastic as the inter-renal body, 

 the nervous as a series of paired bodies connected with the intercostal 

 branches of the aorta 1 . 



All Vertebrata possess a paired kidney. This organ with its duct has 

 a complex history. The primitive kidney duct is known as the * segmental 

 duct ; ' it extends from the anterior region of the coelome to the primi- 

 tive cloaca, opening into both. This duct in Ichthyopsida becomes divided 

 except in Cyclostomi (t Teleosteiand Ganoidei\ see general account of Pisces) 

 into two ducts, a Miillerian duct with a coelomic aperture, and a closed 

 Wolffian duct. In the Amniota the two appear to develope partially, or in 

 some cases perhaps wholly, independently of each other. With the fore 

 part of the segmental duct is connected in the embryoes or larvae of Ichthy- 

 opsida except Elasmobranchii a pronephros. It has the form of 1-5 tubes 

 produced from the extremity of the segmental duct, inclosed in a special 

 section of the coelome, with a vascular ridge or glomerulus opposed to the 

 apertures of the tubes. It appears to atrophy in all cases. There is a 

 somewhat similar structure in the Chick, connected however with the 

 Miillerian duct and devoid of a glomerulus. A mesonephros, or Wolffian 

 body, is developed in all Vertebrata posteriorly to the pronephros, but it is 

 only an embryonic organ in the Amniota with the exception of its genital 

 region in the male (infra). It consists of a series of tubules derived from 

 the peritoneum or mesoblast opening into the segmental duct in Cyclostomi ; 



1 Weldon believes that the supra-renal bodies represent the pronephros in Cyclostomi ; the pro- 

 plus part of the meso-nephros in Teleostei ; a part of the mesonephros in secondary connection with 

 the sympathetic ganglia in Elasmobranchii and higher Vertebrata. P. R. S. xxxvii. 1884 ; see also 

 Q. J. M. xxiv. 1884. 



