THE ELASMOBRANCH FISHES 297 



fig. 254a), where some of them are two to three and a lialf millimeters in 

 diameter; or they may be small as in Scyllium. Frequently when funnels are 

 present in the embryo they become rudimentarj^ or wholly absent in the adult 

 (Raja). When the ne])hrostonies are present they may be traced along the 

 mesorectal mesentery and the mesenteries proper, the most anterior of which 

 reach the mesenteries of the sex glands. The number of nephrostomes varies 

 according to the species, the individual, the sex, and the age. In a type like 

 Squat ina (fig. 254a) there are usually nineteen or twenty pairs present, but 

 these may be reduced in number. In the female the number is always smaller 

 than in the male. Twenty-three or twenty-four pairs are present in the adult 

 Acanthias; while in the embryo there may be as many as thirty-five pairs. 



The segmental ducts lead from the funnels (nephrostomes) outward toward 

 the kidney tissue. These may be clearly marked as in Squat ina or they may 



Fig. 256. Diagram of development of segmental ducts and their relation to the Wolffian 

 and Miilleriaii duets in the embryo of Scyllium. (From Balfour.) 



fl., funnel; )i2)h., nephrostome; o.d., Miillerian duet (oviduct); s.d., segmental duct; 

 w.d., Wolliian duct. 



partly degenerate so that the nephrostome is sessile (ScijlliuDi). In some 

 other types no trace either of the ducts or of the funnels remains in the adult. 

 In those types in which they are developed, the ducts may run more or less 

 directly outward in the midbody region (Squatiua), or they may be V-shaped 

 with the apex pointing forward {Squalus sucklii) . In the region of the rectal 

 gland they may be more difficult to see by reason of the thickness of the meso- 

 rectum. Here in general they extend obliquely backward and may be long 

 drawn out (Squatina) or short as in Scyllium. As the segmental ducts pass 

 outward they pass above the AVolffian ducts {Acanthias) . 



It was formerly supposed that the segmental ducts were directly continued 

 into collecting tubules of the urinary system. If this were so it would be 

 possible for waste substances to be collected from the body cavity and passed 

 out through the Wolffian duct or ureter to the exterior (see fig. 256, Scyllium). 

 Such a connection between the two systems, however, probably does not exist 

 for the adult of any forms since in the adult the segmental duct ends blindly as 

 the median vesicle ventral to the kidney tissue (see Squatina, fig. 254a, m.v.) . 



Proof that no connection exists between segmental duct and collecting 

 tubule has been beautifully shown by the experiments of Schneider (1897) 

 who injected India ink mixed with carmine into the body cavity of the living 

 Squatina. If a connection exist between the nephrostomes of the body cavity 

 and the kidney tissue the ink should be eliminated to the exterior through the 

 Wolffian duct or through the ureters. Upon killing the fish a few days after 

 the experiment Schneider found that the ink and grains of carmine had col- 



