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,[[255]] and possibly also in some Salmonidae.[[256]]
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 Elasmobranchs, 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,[[257]] but, from what has been said as to the structural variability of the valve, it is obvious that its efficacy from a functional standpoint must be equally variable. The value of the valve as an absorptive mechanism necessarily depends on the area 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 liable 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, 136.64; B, 143.82; C, 254.3; and D, 276.7.[[258]] 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 difference in physiological value between the two types must be far more considerable than is indicated by a comparison of their relative superficial areas alone.
The evolution of the spiral valve was probably due to the necessity of increasing the absorptive area of an almost straight unconvoluted intestine, a result which in other animals is often obtained by an increase in the length and concurrent convolution of the intestine itself. Any attempt to correlate the variations in the degree of perfection or imperfection of the valve considered as an absorptive mechanism with any special variations in the nature or quality of the food is, however, a very difficult problem, and a satisfactory explanation has yet to be found. The difficulty, moreover, is increased by the fact that the majority of Fishes with a spiral valve are mainly carnivorous; the Elasmobranchs, in which this structure is at the same time most highly developed and most variable, exclusively so. On the other hand, the term "carnivorous" covers a multiplicity of minor differences in the nature and relative digestibility of different forms of animal food, and it is quite possible that it is with differences of this kind that the specific or individual variations in the development of the spiral valve are associated. The absence of the valve in the variously nourished Teleosts, save perhaps as a vestige in one or two, is also difficult to account for, although it is not improbable that compensating structural modifications exist in this group. As a rule, the intestine is much more convoluted in these Fishes, but to an extent which varies greatly in different species, while the characteristic pyloric caeca and the spiral valve appear to a certain extent to be developed in inverse proportion to one another.
The Glands.
The glands associated with the alimentary canal in different Fishes are (1) the gastric glands, (2) the liver, (3) the pancreas, (4) the pyloric appendages, and (5) the "rectal" gland.
Oral salivary glands are wanting in all Fishes, the only secretory structures in the mouth being numerous mucus-secreting goblet cells, which here, as elsewhere throughout the alimentary canal, are intermixed with the ordinary epithelial cells.
The Gastric Glands.—The Cyclostomata and Dipnoi do not possess any specially differentiated gastric glands, and it is probable that in these Fishes the secretion of the digestive fluids is effected by the ordinary lining epithelium of the stomach or intestine, or both. In the remaining groups gastric glands are generally present in the form of simple caecal structures embedded in the submucosa and opening on the surface of the mucous membrane into the cavity of the stomach. The glands differ in different Fishes in the character of their lining epithelium and in the extent to which their component cells are differentiated from the epithelium of the stomach. There does not appear, however, to be any distinction into "central" (pepsin-forming) and "parietal" (acid-secreting) cells, as is the case in the higher Vertebrata. Towards the pyloric end of the stomach the true gastric glands are often replaced by mucous glands. There are, nevertheless, not a few Teleosts in which special gastric glands are absent, as, for example, Syngnathus acus, and several species of Cyprinidae, Labridae, and Blenniidae, etc. In at least two genera (Gastrosteus and Cobitis), belonging to widely different families, gastric glands are present in certain species but absent in others. As suggested by Edinger,[[259]] the absence of these glands may possibly be due to degeneration.
It may be remarked that the formation of such digestive ferments as pepsin and trypsin, which are associated with the stomach and pancreas respectively, in the higher Vertebrates, is not nearly so strictly localised in Cyclostomes and Fishes. So far from peptic digestion being limited to the stomach, it may take place in the pharynx, stomach, and intestine of Ammocoetes, and in some Elasmobranchs (e.g. Scyllium), and in such Teleosts as the Pike, Eel, and Carp, the peptic region extends from the stomach for some distance along the intestine, while trypsin has been obtained from the mucous membrane of the stomach, intestine and pyloric caeca, as well as from the pancreas.[[260]]
Intestinal glands analogous to the glands of Lieberkühn in the higher Vertebrates seem to be entirely wanting in Fishes, unless represented by the sac-like or tubular crypts which are so generally present in the Teleostomi.