54 MR. T. J. PARKER ON THE INTESTINAL 
valve as that shown in fig. 3, Pl. XI., in which there is again a “nest” of cones, but 
with their apices directed forwards instead of backwards, and with their altitudes 
approximately equal. All the turns have, indeed, the form and direction which is 
normal to the first, and which it retains even in extreme cases of backward deflection. 
§ 7. There are thus four chief forms assumed by the spiral valve of Raia, the cha- 
racters of which may be recapitulated as follows :— 
Type A. Simplest form: free edge in all the turns, but the first on the same level 
as the attached edge; width of valve not greater than half the semidiameter 
of the intestine. This form is more or less hypothetical, the nearest undoubted 
approach to it being that shown in fig. 1, Pl. X. 
Type B, Regular spiral: the free edge is again on the same level as the corre- 
sponding portion of the attached edge ; but the width of the valve is equal to 
the semidiameter of the intestine, so that its thickened free edge forms a 
columella (fig. 4, Pl. X.). 
Type C. All the turns but the first are deflected backwards, the width of the valve 
becoming much greater than the semidiameter of the intestine (fig. 8, Pl. X., 
and fig. 1, Pl. XI.). 
Type D. All the turns deflected forwards, the width of the valve again being con- 
siderably greater than the semidiameter of the intestine (fig. 3, Pl. XL). 
It will be seen that, of these four types, A forms a starting-point or stem form, from 
which the other three may be supposed to have diverged by the turns of the valve 
growing either directly inwards (B), or backwards (C), or forwards (J) during their in- 
crease in width. And this view is borne out by the occurrence of intermediate stages. 
I have found one or two undoubted transition-forms between A and B; and by far the 
greater number of the valves examined are intermediate either between d and C or 
between A and D. 
§ 8. I have figured two of these intermediate forms, figs. 6 & 7, Pl. X., which show 
in a very striking manner that the variations in the valve are quite independent of 
species or sex. Both these specimens, in fact, were taken from adult male examples of 
R. maculata of as nearly as possible the same size. 
To show that the variations are wholly independent of the age of the animal, I may 
mention that although the simplest form (fig. 1, Pl. X.) is from a small specimen, yet 
one of the most complicated (fig. 8, Pl. X.) is from one very little larger, while a far 
simpler form is often seen in full-grown animals. Moreover I have examined two or 
three specimens of Ray not more than three inches long, excluding the tail, and found 
that in them the valve exhibited all the variations occurring in the adult, one approach- 
ing very closely to type C, another to D. 
§ 9. It will at once be perceived that the variations just described in the form of the 
spiral valve will produce a corresponding difference in the amount of absorption-surface 
