. Chordate morphology. Morphology (Animals); Chordata. 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.
. Chordate morphology. Morphology (Animals); Chordata. 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- duc
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