SPIRAL VALVE IN THE GENUS RAIA. 58 
the axis, and become a straight line, thus giving a sort of columella to the spiral, and 
effectually preventing the passage of food in any but a spiral direction. 
It is evident that, retaining the same general character as that described, a much 
simpler and less efficacious form of valve would be produced if the free edge, still re- 
maining parallel to the attached edge, formed a more open spiral. This condition of 
things is well shown in the last four turns of the valve in fig. 1, Pl. X. (the anterior 
turns have undergone a modification presently to be described), the width of which is 
not greater than half the semidiameter of the intestine; and, in consequence, a wide 
passage is left down the axis of the latter, along which its contents could pass directly, 
or without taking a spiral direction at all. It is a valve of this sort which Perrault 
figures in his “ Renard marin” (Alopecias vulpes), and which he aptly compares to “ un 
escalier tournant sans noyau.”’ 
But in this valve (fig. 1)—the simplest I have met with, except one unreliable dried 
specimen—the anterior turns do not retain the simple character described. They have, 
in fact, undergone a notable increase in width, the second turn becoming as wide as 
the intestine itself, and, being now prevented from taking a transverse direction, have 
become more or less folded and deflected, the direction of the deflection being, on the 
whole, a backward one. Thus the inner edge of any given turn of the valve, instead 
of being in the same plane as its attached edge, comes to be in the same plane as the 
attached edge of the turn next behind, or even as that of the next but one. In fig. 1, 
for instance, the free edge of the fourth turn is on the same level as the attached edge 
of the sixth. 
This state of things is carried to an extreme in the cases shown in fig. 8, Pl. X., and 
fig. 1, Pl. XI. In these the width of the second turn is so great that its free edges come 
to be on about the same level as the attached edge of the seventh ; and the width of the 
valve gradually diminishing from the third to the seventh turns, the free edges of these 
also are brought approximately into the same plane. This is well shown in the cross 
section, fig. 9, Pl. X. The successive turns therefore form, as it were, a “nest” of 
imperfect truncated cones, placed, with their apices directed backwards, one inside the 
other, and gradually diminishing in height from the innermost to the outermost. 
Moreover the successive ‘“‘ cones” adhere so closely to one another that there is prac- 
tically produced a central conical chamber, with a peripheral spiral cavity wound round 
it. This condition is, in fact, actually produced if such an intestine be dried; for then 
the successive turns become completely fused together wherever they are in contact. 
In the diagram of this type of valve the free edge is seen to form a very curious and 
complicated figure (fig. 2, Pl. XI.). 
In fig. 1, Pl. X., it will be seen that the first two turns, although on the whole 
deflected backwards, turn at first forwards or towards the pylorus. If this anterior 
deflection were persevered in, and took place in every turn, we should have such a 
