. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Release of Energy 397 Xenopus, corresponding physiologically to the atrial pore in the peribran- chial chamber of amphioxus (Fig. 13). One of the tropical limbless amphibians, Caecilia, is exceptional, having larval external gills of peculiar crumpled leaflike structure, with a relatively large respiratory surface (Fig. 329). 8. Gill Structures in Land Vertebrates The persistence of branchial remains in land forms, that have no use for gills even in em
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Release of Energy 397 Xenopus, corresponding physiologically to the atrial pore in the peribran- chial chamber of amphioxus (Fig. 13). One of the tropical limbless amphibians, Caecilia, is exceptional, having larval external gills of peculiar crumpled leaflike structure, with a relatively large respiratory surface (Fig. 329). 8. Gill Structures in Land Vertebrates The persistence of branchial remains in land forms, that have no use for gills even in embryonic life, is striking evidence of the common ancestry of all vertebrates. Although gills are never present in reptiles, birds, or mammals, there are five pairs of embryonic visceral pouches and furrows in reptiles and mam- mals and four pairs in birds. Ordinarily these break through only briefly in reptiles and birds but not at all in most mammals. Only the anteriormost, or hyomandibular, pair remains well developed in adult mammals. The hyomandibular pouches become the Eustachian tubes and the middle ear cavity, which ordinarily remain separated by the ear drum from the external ear canal, derived from the ectodermal furrows (Fig. 692).. III. SWIM BLADDER A swim bladder, or air bladder, is found in most fishes. It is a derivative of the anterior region of the digestive tube and, if not primarily respiratory in function, is at least found in suspi- cious intimacy with respiratory organs. Located dorsally in the body cavity just outside of the peritoneum under the vertebral column (Fig. 330), it is ordinarily a single elongated struc- ture, although it may be bifurcated or paired, as for example in Polypterus and Calamoichthys among ganoids and in the swellfish, Sphacroides, the curious headfish, Alola, and the sea robin, Prionotus, among teleosts. Great variation in the shape, form, and size of the swim bladder is con- nected with its hydrostatic function, as well as with v
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