766 COMPARATIVE ANATOMY 



structure seeming to anticipate the parabronchi the small uniform sized 

 air-tubes in the lungs of birds which connect the larger secondary 

 branches of the bronchial tubes. "This resemblance is heightened by 

 the development in these same lizards of long, thin-walled sacs from 

 the posterior part of the lungs wh-ich extend among the viscera, even 

 into the pelvic region." The air sacs are used to inflate the body. It is 

 well to remember what has just been said in the study of similarly named 

 structures in the bird. In turtles and crocodiles there is no atrium and 

 the whole lung has a spongy texture. The bronchus in turtles enters 

 on the ventral side of the lung and not as in lizards in the medial. 



SUMMARY OF THE RESPIRATORY SYSTEM 

 BALANOGLOSSUS 



The pharyngeal clefts take the form of gill sacs, each of which opens 

 into the pharynx in a U-shaped slit, resembling that of Amphioxus, and 

 opens to the exterior by a small pore. These gill-slit openings to the 

 pharynx are supported by thin, chitinous bars resembling the gill bar 

 system of Amphioxus. 



FISHES 



The characteristic respiratory organs of aquatic vertebrates are gills 

 or branchiae. Gills are finely divided comb-like outgrowths of the ecto- 

 dermal or endodermal epithelium lining the branchial clefts. The num- 

 ber of clefts or gill slits vary from five to seven in number. Each cleft 

 is separated from its neighbor by branchial septa. The more primitive 

 the fish, the larger number of branchial clefts it is likely to have. The 

 modern types have regularly five. Heptanchus, sometimes mentioned 

 as the most primitive living species of shark, has seven clefts, while 

 Hexanchus, another primitive shark, has six. and elasmobranchs in gen- 

 eral have five fully developed clefts and a vestigial anterior first cleft 

 called a spiracle. 



The spiracle is the rudimentary first cleft, which is also found 

 among the most primitive teleostomi (Crossopterygii and Chondrostei). 

 It is present in the embryos of Teleostei and Holostei, although here it 

 is closed before hatching. In the Holocephali, an aberrant group of 

 elasmobranch fishes, the fifth cleft is closed in the adult, which reduces 

 the number of functional clefts to four. The cyclostomes have on the 

 whole a larger number of clefts than the true fishes. However, the hag- 

 fishes (Fig. 366) of the family Myxinidae have no more than six pairs, 

 while those of the family Bdellostomidae (Fig. 366) have as many as 

 fourteen pairs, and the lampreys all have seven pairs. 



The direction of change in fishes appears to be one of reduction in 

 the number of clefts from fifty or more in Amphioxus (Fig. 437) and 

 Ascidians (Fig. 313), fourteen to six in the cyclostomes, seven to five in 

 the true fishes, and four in the Holocephali. 



