The pelvic plexus of the Phaneroglossa is formed in _Rana_ by the VIII. + IX. + X. + XIth nerves, the tenth issuing between the sacral vertebra and the coccyx. In _Bufo_ and _Hyla_ the plexus is composed of five nerves, the seventh spinal sending a branch to it. Occasionally the twelfth nerve contributes a small branch to the posterior portion of the plexus. This and the eleventh nerve leave the coccyx by separate holes, thereby indicating its composition. The rest of the spinal cord gives off no more recognisable nerves, owing to its reduction during the later stages of metamorphosis; its terminal filament passes out of the posterior end of the coccygeal canal. Concerning the CRANIAL NERVES it is necessary to draw attention to one point only. The last nerve which leaves the cranium of the Amphibia is the vagus or tenth cranial nerve. There is consequently no eleventh, and no twelfth or hypoglossal, pair of cranial nerves. Their homologues would be the first and second spinal nerves, but the whole tongue of the Amphibia, with its muscles, is supplied by the glossopharyngeal, or ninth cranial pair, and is morphologically not homologous with the tongue of the Amniota. {40}RESPIRATORY ORGANS A very important and characteristic feature of the Amphibia is the development of two sets of respiratory organs: Gills and Lungs. It is as well to give definitions of these organs. _Lungs_ are hollow evaginations from the ventral wall of the pharynx, and their thin, vascularised walls enable the blood to exchange, by osmosis, carbon dioxide for oxygen from the air which enters the lungs by the mouth or the nostrils, and the windpipe. The latter is unpaired, the lungs themselves are paired. _Gills_ are highly vascularised, more or less ramified excrescences, covered by a thin epithelium of ecto- or endo-dermal origin, which permits of the exchange of carbon dioxide for oxygen from the air which is suspended in the surrounding water. It is obvious that this definition applies to all sorts of well-vascularised organs whose thin surface comes into contact with the water. Various recesses of the pharyngeal cavity, the dorsal and ventral folds of the tail-fin, nay, even any part of the skin of the body can, and does occasionally, assume additional respiratory functions. The proper definition of gills, in Vertebrates, requires, therefore, the restriction that they must be developed upon and carried by visceral arches.