638 



RESPIRATORY AND BUOYANCY SYSTEMS 



Fig. 300. Respiratory surface relationships in fishes. (A-C original; D and E after 

 Romer: The Vertebrate Body. 1949, Philadelphia, Saunders.) (A-C) External gill 

 filaments and developing gill lamellae on gill arch of shark embryo, Squalus acanthias. 

 (D) Section of gill arch of a shark. (E) Section of gill arch of a teleost fish. 



water-living forms. In reptiles, birds, and mammals, the potency for gill for- 

 mation by these arches ostensibly is lost. 



1. Development of Gills in Fishes 

 a. Development of Gills in Squalus acanthias 



As the developing gill arch of Squalus acanthias enlarges, the lateral por- 

 tion extends outward as a flattened membrane, the gill septum (fig. 300A). 

 On the posterior surface of the early gill arch, the covering epithelium pro- 

 duces elongated structures, the external gill filaments. Each gill filament con- 

 tains a capillary loop which connects with the afferent and efferent branchial 

 arteries (see Chap. 17). These filaments are numerous and give the branchial 

 area a bushy appearance when viewed externally (fig. 300B). The epithelial 

 covering on the anterior face of the gill arch, in the meantime, produces 

 elongated, lamella-like folds, the gill lamellae or gill plates (fig. 300C). During 

 later embryonic life, the external gill filaments are retracted and resorbed as 

 gill lamellae are developed at the basal area of the filaments. The gill arch 

 thus comes to have a series of gill lamellae or plates developed on anterior 

 and posterior surfaces, i.e., the surfaces facing the gill-slit passageway. The 

 gill plates on each surface of the gill arch form a demibranch, and the two 

 demibranchs constitute a holobranch or complete gill. 



Meanwhile, internal changes occur within the branchial arch. The original 

 aortal (vascular) arch becomes divided into efferent and afferent aortal ar- 

 teries, with capillaries interposed between the two (fig. 341A-D). Afferent 

 capillaries bring blood from the afferent portion of the aortal arch to the gill 

 lamellae, while efferent capillaries return the blood to the efferent segment of 

 the aortal arch. Associated with these changes, a skeletal support for the gill 

 arch and gill septum is formed (fig. 315C and D). It is to be observed that 

 the branchial or gill rays extend outward between the lamellae and thus form 

 a series of supports for the gill septum and lamellae. Musculature is developed 

 also in relation to each gill arch (fig. 327B). 



