PHYSOCLYSTOUS 



NO SWIM BLADDER 



Figure 9-34. Possible phylogeny of the swim bladder. 



ond, and third branchial arches and can be observed in the 

 7-mm frog. On each of these arches the external gill is re- 

 duced as the internal gill filaments form along the arch itself 

 The slit between the hyoid and first branchial arch is oblit- 

 erated by the posterior growth of an operculum. The 

 opercula of either side are continuous across the throat. The 

 joined opercula grow back and attach behind the branchial 

 chambers except for a slit on the left side. With enclosure 

 of the confluent branchial spaces, only internal gills are now 

 present. A hemibranch has appeared on the fourth arch be- 

 hind the third and last gill slit; this condition is observed 

 in the 10- mm frog. These gills serve the larva until, with 

 metamorphosis, the lungs come into play. 



Among fossil amphibians only the branchiosaurs and a 

 lew other types such as Dumosaurus retain the gills and visceral 

 skeleton. These types are presumed to be neotemc since 

 the amphibian stage of evolution is defined as having func- 

 tional lungs in the adult. Perhaps only some of the more 

 advanced amphibians, as defined by other criteria, aban- 

 doned gills for lungs. One might assume that Necturus has 

 come down from the beginning of the Amphibia without 

 change in this feature. It is clear in the case of the axolotl 

 that gills are a neotenic feature; Necturus is a parallel case 

 where gills have been retained in the adult as a specializa- 



tion for an aquatic life. Not all aquatic salamanders retam 

 larval gills, as witness Cryptobranchus. 



Choanafe fishes Latimena has a deep spiracular pouch 

 without a mandibular pseudobranch on its anterior wall or 

 an external opening. There is a small hyoidean hemibranch, 

 on the posterior surface of the hyoid arch, and there are four 

 holobranchs, gills along both anterior and posterior margins 

 of an arch (Figure 11-18). The fifth branchial arch is re- 

 duced and without gills. 



The lungfishes, similar in some respects, all lack the 

 mandibular pseudobranch and have only a rudimentary 

 spiracular pouch. Neoceratodus, the most aquatic of the lung- 

 fishes, cannot live out of water. It has an hyoidean hemi- 

 branch and four holobranchs (Figure 11-18). Protoplerus 

 and Lepidosiren, being given to aestivation periods during 

 dry spells, appear to depend more on the lung. Protopterus 

 has the hyoidean hemibranch, the fourth and fifth arches 

 bear holobranchs, and the si.xth has an anterior hemibranch. 

 Vestiges of the external gills of the larva are retained in the 



adult. 



In both Protopterus and Lepidosiren the first branchial or 

 hyobranchial slit, that between the hyoid and the first 

 branchial arch, is closed. Lepidosiren has holobranchs on 



THE RESPIRATORY SYSTEM • 287 



