. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 438 Biology of the Vertebrates — Urinary Duct. Cloaca -Bladder are formed by the widening or enlargement of the urinary ducts. In many fishes two independent bladders may form, vesica duplex (Fig. 368a), one near the end of each urinary duct, with the two ducts afterwards uniting into a common passage-way of exit; or the two may run together into a common bilobed bladder vesica bicornis (Fig. 368b), as in Lepidosteus and some other ganoids; or finally, the
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 438 Biology of the Vertebrates — Urinary Duct. Cloaca -Bladder are formed by the widening or enlargement of the urinary ducts. In many fishes two independent bladders may form, vesica duplex (Fig. 368a), one near the end of each urinary duct, with the two ducts afterwards uniting into a common passage-way of exit; or the two may run together into a common bilobed bladder vesica bicornis (Fig. 368b), as in Lepidosteus and some other ganoids; or finally, the two excretory ducts may first join and then expand into a single bladder, vesica simplex (Fig. 368c), as for exam- ple, in the pike, Esox. In all of these cases the urinary ducts enter at one end of the bladder, while the exit is made at the opposite pole. The tubal bladder, which is frequently larger in the female than in the male, is most common in certain teleost fishes like the Pleuronectidae that have no swim bladder. It is somewhat difficult to account for the presence in fishes of these structures that are rarely absent yet of doubtful utility. The cloacal bladder occurs in dipnoans, amphibians (Fig. 369), and monotremes. It is a diverticulum of the cloacal wall opposite the point where the urinary ducts, with which it has no direct connection, enter. It is located dorsally in lungfishes and ventrally in amphi- bians. In the perennibranchiate amphibians it is considerably elongated, but rounded and broadened in frogs and toads. Frequently it is bilobed while in some urodeles, for example Salamandra, Triton, and Eurycea, the lobes are prolonged into hornlike processes. Cloacal bladders are filled by the closure of the outer cloacal sphincter and the backing up of the urinary secretion into them. The allantoic bladder, according to most embryologists, arises from the enlargement of the proximal or basal end of the embryonic allantoic stalk. It is characteristic of mamma
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