(_f_) The air-bladder differs greatly in its degree of vascularity in various Teleosts, as well as in the extent to which its capillary blood-vessels accumulate at special points on the inner surface to form the so-called "red bodies" or "red glands." In some Teleosts the distribution of capillaries is uniform or nearly so; in others, as in the Carp (_Cyprinus carpio_) the vessels are arranged in fan-like, radiating tufts over almost the whole extent of the inner surface; in others again, as in the Pike (_Esox lucius_) the tufts are larger and more definitely localised. A more extreme modification occurs in some of the Physostomi, in which a remarkable concentration of capillaries takes place at one or more points on the inner surface of the bladder, which project into the cavity of the organ in the form of variously shaped blood-red masses. These "red bodies" are essentially retia mirabilia, consisting of masses of interlacing, tightly-packed capillaries with their afferent arteries and efferent veins. The flattened lining epithelium of the bladder is continued over these bodies without undergoing any special modification. In the common Eel (_Anguilla vulgaris_) there are several of these bodies, of which the largest are near the entrance of the pneumatic duct. In the Physoclisti the "red bodies" seem to be replaced by true glands,[351] which nevertheless in appearance closely resemble the former. Some of the Gadidae, such as the Cod (_Gadus {308}morrhua_), the Haddock (_G. aeglefinus_), and the Hake (_Merluccius vulgaris_), have a single large "red gland" projecting into the interior of the bladder from its dorsal or ventral wall (Fig. 184, A). The John Dory (_Zeus faber_) has five such glands, worm-like and curved in shape, with their concavities facing a central point between them (Fig. 184, B). In these Fishes a "rete mirabile" of blood-vessels forms the vascular basis of the glands. The ordinary pavement epithelium of the bladder becomes replaced by faintly granular, columnar, and evidently glandular cells as it passes over the retia mirabilia, and at the same time becomes invaginated into the mass of capillaries in the form of a number of simple caecal glands (Fig. 185). So far as is at present known, the "red glands" are only found in those Teleosts in which the air-bladder has no ductus pneumaticus, whereas in those Fishes which retain the ductus throughout life there are either no special retia mirabilia, or, as in the Eel, only the so-called "red bodies."[352] [Illustration: FIG. 184.—Red glands, A, of the Cod (_Gadus morrhua_), and B, of the John Dory (_Zeus faber_), seen from the interior of the air-bladder. _bv_, Blood-vessels; _r.g_, red glands. (From Swale Vincent and Stanley Barnes.)] {309}(_g_) and (_h_) The structural modifications involved in the connexion of the air-bladder with the auditory organ, and its adaptation for sound-production, as well as its use in respiration, are considered elsewhere.[353]