In a hypothetical simple type of valve, easily derivable from the more primitive type of Petromyzon, it may be conceived that, while not exceeding in width the semi-diameter of the intestine, the valve becomes disposed in several complete and more or less closely approximated spiral turns, the free edge of the valve being on the same level as its attached margin, and leaving an open axial canal along the centre of the gut. The nearest approach to this hypothetical type, which has been compared, not inaptly, to un escalier tournant sans noyau, is perhaps to be found in the Thresher-Shark (Alopecias vulpes).
The structure of the more complicated spiral valves of other Elasmobranchs are well illustrated within the limits of the single genus Raia.
In one specimen of Raia sp. (Fig. 159, A) the last four coils of the valve are similar to those of the hypothetical type, but the more anterior ones, owing to the greater width of the valve, which here exceeds the semi-diameter of the intestine, have their free margins deflected downwards, while that portion of the valve which forms the first half turn is coiled inwards upon itself, so as to form a hollow cone, open dorsally, and having its apex directed forwards. In other examples a further modification is introduced by the increasing width of the valve, which now, throughout its whole length, equals the semi-diameter of the intestine; and by the formation of an axial columella by the thickened free edge of the valve, which is traversed by a central band of unstriped muscle, as well as by the intra-intestinal artery and vein, and takes the place of the central canal of the preceding types. The valve is, however, still regular, and its free margin remains on the same level as the corresponding portion of the attached edge. In other specimens, again, additional complications are introduced by a still further increase in the width of the valve, which now exceeds, often considerably, the semi-diameter of the intestine, and the consequent deflection of the free edge of the valve either forwards or backwards (C and D). As shown in C the valve, in consequence of the backward deflection of its free margin, presents the appearance of a nest of imperfect truncated cones with their apices directed backwards, the successive cones adhering so closely to one another that they combine to form a central conical chamber with a spirally disposed cavity winding round it. In D, on the contrary, the free edge of the valve is deflected forwards, so that, as in C, a nest of cones is formed, but the apices of the successive cones are directed forwards instead of backwards. Notwithstanding these variations in the structure of the valve as a whole, the first coil or half coil nearly always resembles that described in A.
Fig. 159.—Examples of various types of the spiral valve in Elasmobranchs. A, B, C, and D in specimens of Raia spp.; E, in Sphyrna malleus. A, B, and D represents longitudinal sections of the intestine, the ventral portion of the valve being removed. In C successive portions of the ventral wall of the intestine have been cut out. In E the intestine has been opened along the mid-ventral line and its wall reflected to the right and left; the ventral portion of each coil of the "scroll" valve has been removed. In most of the figures the pylorus is shown in the upper part, and the "rectal" gland in the lower. (From T. Jeffery Parker.)
It is obvious that the structure of the valve varies considerably within the limits of the genus, and it may be added that various intermediate types of structure occur between A and B, A and C, and A and D. The individual variations are perhaps even more remarkable, and appear to be quite independent of age and sex. By way of example it may be mentioned that valves approximating to one or other of those represented by C and D occur in different individuals of Raia maculata of the same sex and similar in size, even in young specimens not more than three inches in length.
As regards other Elasmobranchs, the common Dog-Fish (Scyllium canicula)[[244]] has a well-developed spiral valve disposed in twelve coils, which structurally represents a more highly developed example of the type D. The existence of considerable individual variation is nevertheless indicated by the fact that in one specimen examined the valve was intermediate between C and D, five of the eight cones projecting forwards and three backwards. In a specimen of Notidanus sp.[[245]] there were as many as twenty coils, which in disposition were intermediate between B and C, approximating, however, more nearly to B. In a specimen of the Port Jackson Shark (Heterodontus)[[246]] the valve had eight coils, and in structure was also intermediate between B and C, but approached more nearly to C. Some of the Hammer-headed Sharks (e.g. Sphyrna malleus)[[247]] possess a type of spiral valve which differs considerably from any of those hitherto described, and is termed a "scroll" valve (Fig. 159, E). The attached edge of the valve pursues a straight longitudinal course, or at any rate only describes a half turn and back again in passing from the pyloric to the cloacal extremity of the gut. In the middle of its course the width of the valve is about equal to two-thirds of its length, but towards either extremity it gradually diminishes until the free and attached margins meet. The valve thus constituted is rolled upon itself from left to right, the successive coils being comparable to a series of cylinders placed one inside the other, and becoming gradually larger both in length and diameter from within outwards. A similar valve is present in some of the Carchariidae.
In the Holocephali (e.g. Chimaera monstrosa)[[248]] the valve describes only three and a half coils, and is further remarkable in that the attached margin, for a considerable portion of its extent, does not form a regular spiral but describes only a slightly sinuous course. Posteriorly, the valve is more normal, and consists of about two cones with their apices directed forwards.
In the Dipnoi the spiral valve is well developed, and in Neoceratodus[[249]] describes nine coils, and in Protopterus[[250]] six or seven. The structure of the valve in the latter Dipnoid resembles that of Scyllium canicula, except for the smaller number of cones.
In the more generalised Teleostomi the valve is best developed in the Sturgeon (Acipenser) and in Polypterus. In the former[[251]] the valve is restricted to the posterior half of the total length of the intestine, often extending to within an inch of the anal aperture, and describing in its backward course about seven or eight coils. The width of the valve is about equal to the semi-diameter of the intestine, and the thickened free margin forms a well-marked axial columella, round which the cavity of the gut winds, as in the type B, except that the spiral is a more open one. In Polypterus the valve begins close to the solitary pyloric caecum, and for some distance pursues a straight longitudinal course, but eventually forms a few spiral coils, ceasing, however, at a considerable distance from the anus. The evidence afforded by petrified faeces or "coprolites" proves that certain extinct Crossopterygii (e.g. Macropoma, Megalichthys), like their living representative, Polypterus, possessed a spiral valve.[[252]] In Amia and Lepidosteus[[253]] the valve is almost vestigial, being restricted to the terminal portion of the intestine, and is somewhat variable as to the precise number of its coils. In Amia there are nearly four coils, extending over 3 cm., that is less than a tenth of the total length of the intestine, but in some specimens the coils do not exceed two and a half or three in number. Lepidosteus[[254]] has a still shorter valve which, in specimens of 7-10 cm. in length, may not consist of more than three and a half coils, and in much larger specimens may be reduced to less than two coils, a variation which suggests that a reduction takes place in the number of coils as the fish increases in age and size. The structure of the valve in the three last-mentioned genera resembles that described in Acipenser, and in none of them does 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.