640 



RESPIRATORY AND BUOYANCY SYSTEMS 



Fig. 302. External gills. (A after Kerr: Chap. 9, Entwicklungsgeschichte der Wir- 

 beltiere, by Keibel, Jena, G. Fischer; B from Noble, '31; C-E original.) (A) Larval 

 form of Lepidosiren paradoxa. (B) Larval form of Pseiidohranchus striatus. (C, D) 

 Early developmental stages of Necturus maculosus. (E) Gill filaments on gill of adult 

 Necturus. 



considerably from that found in most fishes. In many species, the gill is a 

 columnar musculo-connective tissue structure with side branches, projecting 

 outward from a restricted area of the branchial arch (fig. 302B). Gill fila- 

 ments or cutaneous vascular villosities extend outward from the tree-like 

 branches of the central column. The exact pattern differs with the species. In 

 some amphibian larvae, the gill is a voluminous sac-like affair (see Noble, 

 '31, p. 61). 



As observed in the previous chapter, there are five pairs of branchial-pouch- 

 groove relationships in frogs and salamanders, although six may occur in the 

 Gymnophiona (Noble, '31, p. 159). In the Gymnophiona, also, the first pair 

 of branchial pouches perforates to the exterior for a while during embryonic life 

 and each perforation forms a spiracle similar to that of the sharks and certain 

 other fish. Later it degenerates. In other Amphibia, the first pair of branchial 

 pouches never perforates to the exterior. It is concerned with the formation 

 of the Eustachian tubes, as in most frogs and toads, or it degenerates and 

 eventually disappears. The second, third, fourth, and fifth pairs of branchial 

 pouches perforate variously in different Amphibia. In the frog, Rana pipiens, 

 the second, third, and fourth branchial-pouch-groove relationships generally 

 perforate, and sometimes the fifth does also. In Necturus maculosus, the third 

 and fourth pairs normally perforate. 



