Fishes of the Western Nort/i Atlantic 7 



Batoids in which the forward wing-like expansions of the pectorals have failed 

 to unite with the head are caught occasionally, and monstrosities of this sort have 

 served as the bases for new generic names (pp. 138, 139, 397).'" 



The absence of the anal fin among all batoids is equally a secondary character, 

 for their embryos develop this fin, as do those of Sharks, but they lose it at an early stage 

 in their development.'' 



Spiracular Breathing. It is common knowledge that the spiracles play a much more 

 important role in respiration among most of the batoids than in any modern Shark. 

 In many of the latter, water is taken in solely through the mouth in breathing and 

 chiefly through the mouth even by such of them as inhale somewhat through the spirac- 

 les, for example Squatina. Most of the batoids, however, take in water chiefly through 

 the spiracles. Although it has been observed that Skates (genus Raja) which usually 

 hold the lower surface of the head slightly elevated above the bottom do inhale some 

 water through the mouth, it is likely that Sting Rays, when well buried in the sand, 

 as they often are, take in water through the spiracles alone.'^ But the Devil Rays (Mo- 

 bulidae) respire chiefly or wholly through the mouth, and their spiracles are correspond- 

 ingly small (p. 484). 



The spiracles of a Skate or Ray, breathing undisturbed, open and close at roughly 

 regular intervals, as one can easily see by watching one in an aquarium, closure being 

 effected mainly by the contraction of their anterior margins that bear the rudimentary 

 gill filaments and the spiracular valve. The water that is taken in through the spiracles 

 when the pharyngeal cavity is expanded is prevented from passing out through the mouth 

 by the presence of a broad transverse fold of the oral membrane on the roof of the 

 mouth and by a narrower one on its floor ; the water is thus directed to the gill openings 

 when the pharyngeal cavity is contracted. When a Sting Ray is buried in the sand, 

 the periodic expulsion of water through the gills is made evident by "a regular geyser 

 of sand grains arising to an inch or two in height at the anterior margin of the pectoral 

 fin."" In one set of experiments,'* the rate of respiration in a Skate (Raja), easily mea- 

 sured by timing the contractions and expansions of the spiracles, varied from about 

 30 per minute at rest to 47.5 per minute after exercise. 



It is also well known and easily observed that the direction of flow through the 

 spiracles is occasionally reversed in Skates (Raja), in Sting Rays (Urolophus), in Guitar- 

 fishes (Rhinobatos), and also in Angel Sharks (Squatina). In a Skate at rest this was 

 seen to happen at intervals of five to ten minutes. And it has been shown experimentally 

 that this spouting can be brought about in various ways, i.e., by fatigue, by partial 



paper on the development of Raja. For accounts among Sting Rays, see Hill (Proc. Linn. Soc. N. S. W. [2] 10, 

 1895: 208) and Daniel (Elasmobranch Fishes, 1934: 11, fig. 22, B. C.) for Urolophus. 



10. For a good general account with illustrations, see Gudger (Amer. Mus. Novit., 600, 1933)- 



11. For illustrations showing the anal fin in the embryo Skate, see Wyman (Mem. Amer. Acad. Arts Sci., [N. S.] 9, 

 1867: pi. to face p. 44, figs. 6-9). 



12. See Rand (Amer. Nat., 41, 1907: 287-302), for observations and experiments on the spiracular breathing of Skates 

 {Raja), and Daniel (Elasmobranch Fishes, 1934: 156-157), for similar observations on Sting Rays [Urolophus), 

 Guitarfishes [Rhinohatos), and Angel Sharks [Squatina). 



13. Daniel, Elasmobranch Fishes, 1934: 157. 14. Rand, Amer. Nat., 41, 1907: 293. 



