Scytlium, Squatina, Carchartus, Pnsliiirus, Heptanchiis, and rays. 

 Wassnezow (1928, 1932) found one to three pairs of pits, 

 dorso- and ventrolaterally behind the fifth pouch in Trygon, 

 Torpedo, and Pristiurus. The most posterior of these in Trygon 

 forms a dorsomedial pit comparable to that of Muslelus. In 

 most adult sharks there is no evidence of a swim bladder, 

 and, in the development of the shark, there is only this hint 

 of a rudimentary swim bladder. 



In the fossil antiarch Bothnolefiis canadensis, casts of what 

 appear to be lungs have been described. This interpretation 

 has been objected to on the grounds that these structures lie 

 too far forward and are thus casts of the gill chambers. 



C/c/osfomes In neither of the living cyclostomes is there 

 any evidence of a swim bladder or lungs. Goethe (1875, 1903) 

 observed in the lamprey a small pair of diverticula behind 

 the most posterior gill pouch (the 9th). These were inter- 

 preted as lung rudiments. They might just as easily be in- 

 terpreted as rudimentary gill pouches arising behind the 

 branchial series, as is suggested in the my.xinoids where the 

 number of gill pouches is not a constant. 



General observations There is little question that some sort 

 of homology exists between the lungs of tetrapods and the 

 swim bladders of fishes, but the exact degree is hard to de- 

 termine. One can assume that these structures arose from 

 a posterior pair of pharyngeal pouches, but little more than 

 this. One cannot be sure that the same pair of pouches was 

 involved in every case. It is suggested that in many primi- 

 tive fishes such as sharks these pharyngeal sacs remained 

 rudimentary or disappeared, while in others they became 

 enlarged as a part of a pharyngeal air "breathing" system. 

 In the ancestral actinopterygian the sacs appear to have 

 joined above the esophagus and to have connected with 

 the pharynx, or the esophagus; in choanates and in 

 Polyplerus the sacs joined below and connected ventrally. 

 Thus there may have been at least four lines of evolution: 

 loss, joining above, and joining below in both the choanate 

 and, as a separate event, Polyplerus. The possibility of 

 Polyplerus and the choanate representing a single line is 

 remote since they difl^er in the nature of the "ductus pneu- 

 maticus." The fact that the lungs of some choanates rotated 

 upward around the esophagus, as in the dipnoan, or passed 

 upward to either side suggests the importance of hydrostatic 

 factors in fishes (or amphibians); the resultant dorsal posi- 

 tion is a parallelism to the situation in the actinopterygian. 

 The same events occurred in the evolution of the line lead- 

 ing to Polyplerus. What factors operated to produce a ventral 

 connection are not known; there is the possibility that this 

 is a secondarily achieved relationship. It has also been sug- 

 gested that the dorsal connection of the ray-finned fish may 

 have been derived from an original ventral attachment. This 

 view is popular since it allows a monophyletic origin for both 

 lung and swim bladder. 



Gills 



The gills of vertebrates involve special modifications of 

 the aortic arches which are discussed in Chapter 11. Only 

 a brief summarizing account will be attempted here. 



Amphibians Gills occur only in the larva or as a neotenic 

 feature of the adult in such salamanders as the axoloti or 

 Necturus (Figure 7-23). In the adult oi .Necturus there are two 

 gill slits; these lie between the first and second and the sec- 

 ond and third branchial arches. These gill slits are margined 

 internally by reduced gill rakers. There are no gill filaments 

 on these arches, but there are three pairs of external giUs 

 extending outward from the epibranchial regions of the first 

 three branchial arches. Cryptobranchus has a single pharyn- 

 geal slit between the first and second branchial arches (see 

 Figure 4-18). There are neither internal nor external gills 

 in this species. In the a.xoloti there are four gill slits (I-IV), 

 three external gills in the adult, and an incipient operculum 

 which extends from one side to the other across the throat. 

 In the larval frog three pairs of external gills form (Fig- 

 ure 7-24). The first pair before hatching, the second and 

 third pairs at intervals. These are attached to the first, sec- 



esophagus opened 

 opening of swim bladder- 

 bile duct 



stomach 



aortic arches 



coeliaco-mesenteric artery 

 dorsal aorta 



4— swim bladder 

 interior 



pyloric caeca 



Figure 9-33. Swim bladder and its relationships in Gymnorchus, as 

 seen in ventrolateral view. (After HyrtI) 



286 • THE VISCERA 



