110 ZOOLOGY 



SECT. 



tween it and the cartilage is a space, filled by a fluid called 

 perilyinph, which acts as a buffer to the delicate organ floating 

 in it. 



Urinogenital Organs. In all Craniata there is so close a 

 connection between the organs of renal excretion and those of 

 reproduction tha.t the two systems are conveniently considered 

 together as the urinogenital organs. 



Speaking generally, the excretory organ consists of three parts, 

 all paired and situated along the dorsal wall of the ccelome ; the 

 fore-kidney or pronephros (Fig. 715, A, p. nph.), the mid-kidney or 

 mesonephros (ms. nph.), and the hind-kidney QT metanephros (mt. nph.). 

 Each of these is provided with a duct, the pro- (pn. d.), meso- 

 (msn. d.), and meta-nepliric (mt. n. d.) ducts, which open into the 

 cloaca. The gonads (gon.) lie in the coelome suspended to its dorsal 

 wall by a fold of peritoneum : in some cases their products are 

 discharged into the coelome and make their exit by abdominal 

 pores, but more usually the pronephric duct in the female 

 assumes the functions of an .oviduct and the mesonephric duct in 

 the male those of a spermiduct. The pronephros is almost always 

 functionless in the adult, and usually disappears altogether. The 

 mesonepliros is usually the functional kidney in the lower Craniata, 

 in which, as a rule, no metanephros is -developed, and the mesone- 

 phric duct, in addition to carrying the seminal fluid of the male, 

 acts as a ureter. In the higher forms the mesonephros atrophies, 

 and the metanephros is the functional kidney, the metanephric 

 duct becoming the ureter. 



The kidney meso- or meta-iiephros of the adult is a massive 

 gland of a deep red colour made up of convoluted urinary tiibulcs 

 (Fig. 746), separated from one another by connective tissue con- 

 taining an abundant supply of blood vessels. The tubules are 

 lined by a single layer of glandulajj epithelial cells (B, C) and 

 each ends blindly in a globular dilatation, the Malpigliian capsule 

 (A, gl.), lined with squamous epithelium. In many of the lower 

 Craniata, a branch goes off from the tubule, near the Malpighian 

 capsules, and, passing to the ventral surface of the kidney, ends 

 in a ciliated funnel-like body (Fig. 747, nst.), resembling the 

 nephrostome of a worm, and, like it, opening into the coelome. 

 At their opposite ends the tubules join with one another, arid 

 finally discharge into the ureter. 



The renal arteries branch extensively in the kidney, and give 

 off to each Malpighian capsule a minute (Afferent artery (Fig. 746, 

 A, v. a.} : this pushes the wall of the capsule before it, and breaks 

 up into a bunch of looped capillaries, called the glomerulus, sus- 

 pended in the interior of the capsule. The blood is carried off 

 from the glomerulus by an efferent vessel (v. c.\ which joins the 

 general capillary system of the kidneys, forming a network over the 



