DEVELOPMENT OF GILL RESPIRATORY ORGANS 637 



portant in the amphibian group as a respiratory mechanism (Noble, '31, pp. 

 162, 174-175). However, considering the vertebrate group in its entirety, 

 the branchial or pharyngeal area is the particular part of the developing body 

 devoted to the formation of adult respiratory mechanisms. 



c. Main Types of Organs Used for Respiration 

 Two main types of respiratory organs are developed in the vertebrate group: 



( 1 ) branchial organs or gills in water-living forms and 



(2) pulmonary organs or lungs in land-frequenting species. 



Both of these organs represent pharyngeal modifications. 



B. Development of Branchial or Gill Respiratory Organs 



As observed in the previous chapter, p. 618, the invaginating branchial 

 grooves and the outpocketing branchial pouches come together in apposition 

 in the early embryos of all vertebrate species, and, in water-living forms, 

 varying numbers of these pouch-groove relationships perforate to form the 

 gill slits. In cyclostomatous fishes (fig. 301 A, B), the number of perforations 

 is six or more pairs; in elasmobranch and teleost fishes, there are five or six 

 pairs (fig. 301C, D); and in amphibia, two or three pairs become perforated. 

 In general, the first pair of branchial-pouch-groove areas is concerned with 

 the formation of the spiracular openings or with the auditory mechanisms. 

 However, in some species it may be vestigial. In water-inhabiting species, the 

 succeeding pairs of pouch-groove areas and their accompanying visceral arches 

 may develop gill structures. (See p. 669, visceral skeleton.) 



Two types of gill mechanisms are developed in the vertebrate group: 



( 1 ) internal gills in fishes and 



(2) external gills in amphibia and in lung fishes. 



In all cases, gill development involves a modification of visceral-arch struc- 

 ture. This modification involves the external surface membranes and blood 

 vessels of the arches. The first two pairs of visceral arches, the hyoid and 

 mandibular, are utilized generally throughout the vertebrate series in jaw and 

 tongue formation (see Chap. 13). On the other hand, the third and succeeding 

 pairs of visceral arches are potentially branchial or gill-bearing arches in 



Fig. 299. Structural relationships of respiratory surfaces. (A after Clements, '38; B 

 after Noble, '31; E after Patten: Am. Scientist, vol. 39, '51; F and G after Noble, '25; C 

 and D original.) (A) Respiratory surface in air sac of pig, 18 hrs. after birth. Capil- 

 laries are exposed to air surface. (B) Section through epidermis of respiratory, integu- 

 mentary folds along the sides of the body of Cryptobranchus alleganiensis. (C) Trans- 

 verse section of external gill filament of Rana pipiens. (D) External gill filaments of 

 Squalus acanthias. (E) The ailantoic-egg-surface relationship of the developing chick 

 embryo. (F) Respiratory villosities or "hair" of Astylosternus robust us, the hairy 

 frog. (G) Section through skin of vascular villosity shown in (F). 



