Chondrichthyes 



Shark The kidney of the shark is an opisthonephros, a long 

 band increasing in dimensions posteriorly (Figure 10-48). 

 Its surface is irregular and in some species distinctly lobed. 

 In the sexually mature male, the anterior part is converted 

 into an epididymis. The nephric duct becomes hypertrophied 

 and very looped and coiled, acting as a ductus deferens — the 

 anterior part of the nephric duct of the female shows a tend- 

 ency in this direction. Posterior to the tubular epididymis, 

 a part of the kidney of the male is modified into the Leydig's 

 gland. Behind this is the expanded excretory part of the 

 kidney. The kidneys of either side are in contact posteriorly 

 but not joined. 



The nephrons of the epididymis portion each have a renal 

 corpuscle and a coiled tubule. With establishment of con- 

 nection with the testis, the renal corpuscle is lost and the 

 lumen of the tubule is increased. The Leydig's gland por- 

 tion is formed of functional nephrons in the young, but 

 these become aglomerular and secretory in the adult. They 

 produce a secretion which acts in the formation of sperma- 

 tophores (bundles of sperm) and in the transport and 

 conditioning of the sperm. In the female, with the exception 

 of the extreme anterior end which degenerates (epididymis 

 area), the entire opisthonephric band is functional. 



The nephric duct in some sharks (Squalus) is converted 

 into a vas deferens in the male, and the posterior excretory 

 part of the kidney is drained by a separate duct, the ureter. 

 All stages of separation of vas deferens and ureter are shown 

 among sharks. Some species have several ureters opening 

 into both nephric duct and cloaca. The least modification is 

 observed in females of Squalus where segmental ducts from 

 the entire length of the definitive kidney enter the nephric 

 duct; posteriorly one or two segments may be drained by a 

 separate ureter. In some sharks (Torpedo) the female is like 

 the extreme of the male; the anterior Leydig's part is non- 

 functional and attached only to the nephric duct, while the 

 separated posterior part of the opisthonephros is expanded 

 and lobate, and is drained by a separate ureter into the 

 cloaca. 



Peritoneal funnels are present throughout life in some 

 sharks. These have been related to the presence or absence 

 of abdominal pores — the funnels act as a substitute for these 

 pores. In some sharks these funnels are present only in the 

 young. Squalus has such tunnels throughout life along with 

 abdominal pores. These funnels can be demonstrated by 

 staining with Flemming's fluid, a mixture of chromic, acetic, 

 and osmic acids. This is allowed to stand in the abdominal 

 cavity for a time, then flushed out. The funnels are stained 

 by an accumulation of osmic acid and lie near the midline 

 or on the mesenteries which anteriorly support the gonad 

 or posteriorly the rectum. 



The peritoneal funnels of both the Leydig's region and 

 functional excretory region lead into a mass of lymphoid 

 tissue and end at a vesicle. The funnels are not connected 



with the nephrons. This is probably true even in the case of 

 Scylhum. The lymphoid tissue of the kidney may be well 

 marked as segmental patches around the lateral vesicles of 

 the funnels (Squatina) or may diffuse within the kidney sub- 

 stance. 



EMBRYOLOGICAL DEVELOPMENT The pronephros of the 

 shark is reduced and never functional. As a general pro- 

 position, the pronephros of embryos with large yoke masses 

 is never functional, at least in terms of fluid passing down 

 the nephric duct to the cloaca — see myxinid. 



The number of pronephric rudiments varies from four to 

 eight and these lie in metotic segments 7 to 10 or 7 to 14. 

 In Squalus there are about six somites involved (7 to' 12). The 

 nephrotomes (intermediate plate area) of these segments 

 develop nephrocoels which connect with the coelomic spaces 

 laterally. The roofs of these nephrotomes are evaginated up- 

 ward as funnels and the pronephric crests (Figure 10-24). 

 The thickenings join together to form a ridge which hollows 

 out as a pronephric duct. Funnels are formed which open 

 into the coelom. Glomeruli are lacking. 



The nephric duct grows posteriorly from the pronephric 

 region without drawing materials from the overlying ecto- 

 derm or the underlying splanchnopleure (or nephrotome) 

 and thus the shark resembles the reptiles and mammals. 

 The pronephric funnels tend to fuse and form a single large 

 funnel on either side. In Prisliurus or Squalus these bilateral 

 funnels move downward over the wall of the gut, meet be- 

 low the gut, and fuse. This median funnel (or bilateral fun- 

 nel) is part of the definitive MuUerian duct system of the 

 reproductive apparatus. In some sharks, only a single pro- 

 nephric funnel appears to be involved in the ostium of the 

 Mullerian duct. There is a distinct resemblance here be- 

 tween shark and dipnoan. 



The mesonephros extends back from the pronephric region 

 through 35 to 37 somites (37 pairs of canals in Squalus). In 

 Squalus there is a tubule in each segment built of two com- 

 ponents. The first is a peritoneal funnel leading into a tu- 

 bule which arches up and laterally; this represents the 

 nephrotome opening into the coelom. The second part is an 

 outgrowth from the nephric duct toward the blind end of 

 the first; this is a collecting tubule. The tubules develop, as 

 a generality, in an anterior-posterior sequence; the most an- 

 terior ones, however, are somewhat behind those further 

 back and do not develop very far. The first seven or eight 

 degenerate, as may the most posterior ones. There are 25 to 

 30 segments in the definitive kidney (23 to 24 pairs of 

 peritoneal funnels in Squalus). The most complex tubules 

 and the greatest number of generations of tubules develop 

 in the posterior half 



As the peritoneal funnel tubules develop, the middle sec- 

 tion of each expands and separates into a rudimentary cap- 

 sule, for the distal part, which is now a first order nephron, 

 and a sacculate lateral vesicle, which can be identified as 

 the nephrocoel (Figure 10-50). This division is followed by 



310 



THE UROGENITAL SYSTEM 



