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,' and possibly 
also in some Salmonidae.” 
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,’ 
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 lable 
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, 156°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, Ub. d. Darmkanal u. d. Zeugungsorgane d. Fische, Halle, 1824, pp. 62 f. 
3 Edinger, op. cit. p. 678. 4 T. Jeffery Parker, op. cit. p. 55. 
