2s2 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tary canal, which thus serves for the passage of food and for the per- 

 formance 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 

 connexion must be considered the cases of certain fishes, amphibia, 

 and turtles where respiration is effected in part by the skin, the 

 pharyngeal epithelium, or the digestive tract. There are also a 

 number of other structures — air bladder, thymus and thyreoid 

 glands, etc., which are derived from the pharynx, though they are 

 without respiratory functions. 



GILLS OR BRANCHIiE 



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 



Fig. 271. — Two stages in the development of the gill-clefts of Ceratodus, after 

 Greil. Entoderm, black; mesoderm stippled. 0, operculum; oe, oesophagus; ra, 

 radix aortae; i, 2, first and second somites of Van Wijhe; I-V, visceral pouches, not yet 

 broken through. Noticeable are the ingrowths of ectoderm in the line of the future 

 clefts. 



sides of the pharynx. These clefts arise as paired pouches or grooves 

 of the entoderm of the pharynx (figs. 232, 271). They extend later- 

 ally, 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 cavity of the pharynx is connected 

 with the exterior by a series of openings (fig. 270), the clefts de- 

 veloping in succession from the cephalic end backward. 



