IX SPIRAL VALVE 269 



the width of the valve so far exceed the semi-diameter of the 

 intestine as, by forward or backward deflection, to give rise to 

 the highly characteristic cones of Elasmobranchs and Dipnoi. 



In the more specialised Teleostomi (Teleostei) the spiral valve 

 is wholly wanting, except perhaps as a vestigial structure in 

 certain Clupeoids, as, for example, Chirocentrus, 1 and possibly 

 also in some Salmonidae. 2 



From what has been said as to the structure of the spiral 

 valve in the different groups of Fishes, it may be concluded that 

 the valve most nearly retains its primitive condition in the 

 Cyclostomata ; attains its maximum development in the Elasmo- 

 branchs, especially in the Notidanidae, and shows no indication 

 of degeneration in the Dipnoi. In the Holocephali and the lower 

 Teleostomi, on the other hand, the valve exhibits various stages 

 of retrogressive modification, and in the Teleosts is either absent 

 altogether or persists only as a vestigial structure in a very few 

 species. 



From a physiological point of view the object of the spiral 

 valve is to increase the absorptive inner surface of the intestine, 3 

 but, from what has been said as to the structural variability of 

 the valve, it is obvious that its efficacy from a functional stand- 

 point must be equally variable. The value of the valve as an 

 absorptive mechanism necessarily depends on thearea of absorption- 

 surface which it provides, as well as on the degree of resistance 

 which it offers to the passage of food material along the cavity 

 of the intestine. These factors will in turn depend on the number 

 of coils, on the width of the valve, and on the extent to which its 

 free margin is deflected in forming the series of cones, but these 

 again are precisely the structural features which are most liable 

 to variation. The total absorption area in the four types of 

 valve characteristic of the genus Raia has been calculated, and 

 may be expressed in square centimetres as follows: A, 136'64 ; 

 B, 143-82; C, 254'3 ; and D, 276'7. 4 Hence as regards mere 

 absorption area a spiral valve of the type D has twice the extent 

 of a valve of the type A, and if, in addition, account be taken 

 of the retardation of the food due to the increased obstruction 

 offered by the columella and cones in D, it is clear that the 



1 Cuvier and Valenciennes, Hist. Nat. d. Poiss. xix. 1846, p. 151. 



2 Rathke, Vb. d. Darmkanal u. d. Zeugungsorgane d. Fisehe, Halle, 1824, pp. 62 f. 



3 Edinger, op. cit. p. 678. 4 T. Jeffery Parker, op. cit. p. 55. 



