. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. DEVELOPMENT OF GILL RESPIRATORY ORGANS 639 b. Gills of Teleast Fishes Gill development in teleost fishes is similar to that of Squalus acanthias, but the gill septum is reduced, more in some species than in others (fig. 300D, E). An operculum or external covering of the gills, supported by a bony skeleton, also is developed. The operculum forms an armor-like, pro- tective door, hinged anteriorly, which may be opened and closed by opercular muscles
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. DEVELOPMENT OF GILL RESPIRATORY ORGANS 639 b. Gills of Teleast Fishes Gill development in teleost fishes is similar to that of Squalus acanthias, but the gill septum is reduced, more in some species than in others (fig. 300D, E). An operculum or external covering of the gills, supported by a bony skeleton, also is developed. The operculum forms an armor-like, pro- tective door, hinged anteriorly, which may be opened and closed by opercular muscles (fig. 301D). c. External Gills Aside from the formation of external gill filaments as mentioned above (fig. 300B), true external gills, resembling those of Amphibia, occur in most of the dipnoan (lung) fishes and Polypterus in the larval stages (fig. 302A). 2. Development of Gills in Amphibia a. General Features The gills of Amphibia occur only in the larval condition and in some adults which retain a complete aquatic existence, such as the mud puppy, Necturus maculosus, and the axolotl, Ambystoma mexicanum. In other adult amphibia which have not renounced a continuous watery existence, such as Amphiuma and Cryptobranchus, the larval gills also are lost. Cryptobranchus relies largely upon the skin as a respiratory mechanism (fig. 299B). External gills are formed in the larval stage of all amphibia, and, in some, they present a bizarre appearance (Noble, '31, Chaps. Ill and VII). In the frog tadpole, external gills are formed first, to be superseded later by an internal variety. The amphibian external gill is a pharyngeal respiratory device which differs. Fig. 30L Gill arrangement in various fishes. (After Dean: Fishes, Living and Fossil, 1895, New York and London, Macmillan and Co.) (A) Polistotrema {Bdellostoma). (B) Hagfish, Myxine. (C) Shark. (D) Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabil
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