SPIRAL VALVE IN THE GENUS. RAIA. 59 
possess what may be called a scroll valve—that is, a fold of the intestinal wall, the fixed 
edge of which is usually stated to “run straight and parallel with the axis of the intes- 
tine,” while the fold is “ rolled up upon itself into a cylindrical spiral.” 
This description is not strictly correct. On opening the gut along its ventral wall 
by a longitudinal incision (Pl. XI. fig. 8), the valve appears quite like a second intes- 
tine within and nearly filling the first. It may be unrolled from right to left; and 
it is thus seen that the attached edge is not in a straight line, but is slightly (though 
distinctly) curved, beginning just posterior to the pylorus on the right side, curving 
gently outwards until it actually passes to the left of the median ventral line of the 
intestine, and then back again to the right, to end on the dorsal side at the commence- 
ment of the rectum. The free edge is very strongly curved—the width of the valve, in 
the middle being equal to two thirds of its length, while at either end it gradually 
diminishes until the free and attached edges meet. Thus the form of the unrolled 
valve may be compared to that of a vertical section of a biconvex lens one surface of 
which has a very slight, the other a very strong curvature. 
The valve thus constituted is rolled upon itself from left to right, the successive 
turns being comparable to a series of cylinders placed one within the other and be- 
coming gradually larger, in length as well as in diameter, from within outwards. This 
is well shown in fig. 8, in which the ventral portion of each turn is cut away. 
I give this description and the accompanying figures of the scroll valve of Zygena 
for the sake of comparison with the spiral valve; my account adds nothing to Duver- 
noy’s excellent description of the similar valve in Thalassorhinus vulpecula}. 
§ 19. In Lepidosiren I have found the spiral valve to be a well-marked, that ot 
Ceratodus a less perfect, example of type D. To the latter Dr. Giinther’s description? 
applied perfectly well: 
§ 20. Lastly, in the Lamprey there is, as is well known, a ridge of mucous membrane 
projecting into the intestine, round the inner surface of which it takes a spiral course; 
the spiral being a very open one, the whole width of the valve not more than half the 
diameter of the gut. ~The valve is therefore an extremely simple example of type A. 
A valve of this sort is, of course, to all intents and purposes, a typhlosole, only dif- 
fering from the structure of that name in worms from the fact that its course is spiral 
instead of straight—just as the papillose ridges of the hind gut are spiral in Astacus, 
while they are straight in Homarus. Such a valve also bears a close resemblance to 
the embryonic condition of the spiral valve in the Elasmobranch°. 
§ 21. Thus the spiral valve, reduced to its simplest expression, becomes a typhlosole ; 
and the scroll valve, indefinitely reduced in width, becomes the same thing. Even in 
the fully developed structures we get a sort of hint of a connexion between the two; 
1 Ann. des Sci. Nat. 2° série, 1835, t. iii. 
? Phil. Trans. 1871, part ii. p. 511. 
* Balfour, ‘ Elasmobranch Fishes,’ pl. xvii. fig. 2. 
