42 ORGANS OF SECRETION. 



Organs of Srcrrfion. 



We may consider under this head the kidney, mucus gland, 

 purple gland and anal gland : — all special organs situated in the 

 respiratory cavity or opening into it. 



The kidney is a large, hollow, glandular mass at the base of 

 the respiratoiy cavity, close to or sometimes perfoi-ated (Triton) 

 by the rectum. It contains a fluid having a whitish or brownish 

 appearance, filled with hard granules, and in wliicli Jacobson 

 first detected the presence of uric acid, ammouiac and salts of 

 lime. 



If the kidney is cut open the internal cavity is observed, which 

 is, however, much narrowed by numerous thick, spongy, crimped 

 annular folds or meshes, which clothe it internally. The spongy 

 walls, the surface of which, because of the folds is much increased, 

 are covered with round cells, which excrete the urinary products. 

 At the wall of the cells, at least in the youngest ones, there is 

 alwaj^s a distinct nucleus and its contents consist sometimes of 

 a yellowish or greenish fluid, and within are concentricallj' lami- 

 nated urinar}' concretions. By bursting, these secretory cells 

 allow the urinary concretious to escape into the water in the 

 kidney. 



Externally, the kidney is spun over b}'^ a very thick and strong- 

 vascular network of the venous system. This vascular network 

 has several openings into the kidne}^, througli Avhich the urinary 

 products ai'e mingled with the blood ; and, consequent!}", when 

 these are examined microscopically, they are found to contain, 

 besides renal cells and free concretious, a considerabh^ number 

 of blood corpuscles. 



Usually the kidney opens into the base of the jespiratory 

 cavity by means of a transverse, slit-like opening, encircled by a 

 strong sphincter muscle (Triton, Dolium, Cassis, Murex, Lit- 

 torina, Natica, etc.) ; sometimes, however, it has a gut-like, 

 efferent duct, a uretei-, which passes forward between the rectum 

 and sexual canal, opening outwards not far behind the anus 

 (Paludina, Turbo, Yoluta, Conns). There are accordingly tliree 

 eft'erent canals alongside of each other at the right side of the 

 respiratoi'y cavity, viz., the rectum, ureter, vagina or vas deferens. 

 The ureter is usually a somewhat dilated canal and is not simply 



