236 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the vertebrates the organs of respiration are developed in more 

 or less intimate connection with the cephalic portion of the digestive 

 tract, just behind the cavity of the mouth. This part of the alimentary 



canal, which thus serves for the pass- 

 age of food and for the performance 

 of respiratory functions is called the 

 pharynx. The organs themselves 

 may take the form of gills or branchiae, 

 adapted for aquatic respiration, or of 

 lungs (pulmones) fitted for breathing 

 air. In this connection must be con- 

 sidered the cases of certain fishes, 

 amphibia, and turtles where respiration 

 is effected in part by the skin, the 

 pharyngeal epithelium, or the diges- 

 tive tract. There are also a number 

 of other structures air bladder, thy- 

 mus and thyreoid glands, etc., which 

 are derived from the pharynx, though 

 they are without respiratory functions. 



GILLS OR BRANCHIAE, 



The typical gills or branchiae are 

 developed on the walls of some of the 

 visceral clefts (gill or branchial 

 clefts) which are formed in the sides 

 of the pharynx. These clefts arise as 

 paired pouches or grooves of the en- 

 toderm of the pharynx (fig. 208). 

 They extend laterally, pushing aside 

 the mesoderm, until they reach the 

 ectoderm, ectoderm and entoderm 

 then fusing to a plate. This in most 

 cases becomes perforated, so that the 



P 

 oe 



FIG. 243. Pharyngeal region of a 

 young Acanthias embryo, b, blood- 

 vessels; c, ccelornic cavities of gill arches; 

 g, developing gills; gc, gill clefts; h, 

 hypophysis; m, mouth; n, notochord; o, 

 oculomotor nerve; oe, oesophagus; p, 

 peritoneal cavity; s, spiracular cleft; 

 I-III, first to third head cavities. 



cavity of the pharynx is connected with the exterior by a series of 

 openings (fig. 243), the clefts developing in succession from the 

 cephalic end backward. 



These visceral pouches develop in all vertebrates, but in the mam- 

 mals only a few or even none of them break through to the exterior. In 



