DEVELOPMENT OF GILL RESPIRATORY ORGANS 



639 



b. Gills of Te least 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 external gills are formed first, to be superseded later by an internal variety.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. 30 L 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) Teleost. 



