Title: Biology of the vertebrates : a comparative study of man and his animal allies Identifier: biologyofvertebr00walt Year: 1949 (1940s) Authors: Walter, Herbert Eugene, b. 1867; Sayles, Leonard Perkins, 1902- Subjects: Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative Publisher: New York : Macmillan Co. Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image: 438 Biology of the Vertebrates — Urinary Duct

Text Appearing After Image: 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 mammals and of such reptiles as turtles and certain lizards that have a bladder. In the case of other amniotes, like snakes, croco- diles, some lizards, and birds, the whole allantois degenerates without devel- oping a bladder. In mammals that part of the allantoic stalk left within the body wall, when the umbilical cord connected with the placenta is severed at birth, enlarges to form the bladder, and also the urachus, or vesico- umbilical ligament which anchors the bladder to the inner body wall at the umbilicus. Thus the proximal end of the allantois stalk enlarges into the hollow sac of the bladder while the distal part within the body wall under- J v° Fig. 369. Ventral view of cloacal bladder of an amphi- bian.

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