280 CLASS AMPHIBIA. 



them and the external gills. Like the latter they are branched 

 cutaneous structures covered by ectoderm. There is a double 

 row of these gills on each of the first three branchial arches and 

 one row on the fourth. In all the Anura and in most of the 

 Urodela the gUl-clefts close and the gills atrophy when the adult 

 state is reached, but in a few of the Urodela, the so-called 

 Perennibranchiates {Proteus, Necturus, Siren, etc.), some of them 

 persist throughout life (for details, see systematic part). In 

 the Amphiumidae the gills disappear but the last cleft usually 

 persists, and in the genus Amhlystoma the animal sometimes 

 becomes sexually mature in the larval condition and does not 

 metamorphose {Siredon, the axolotl). This phenomenon of the 

 retention of larval characters in the sexualh' mature state has 

 been called neoteny, or paedogenesis ; it is not confined to 

 Amhlystoma but is found occasionally in the genus Triton (Molge). 

 An analogous phenomenon is presented by many tadpoles and 

 by some Urodeles, in which the metamorphosis is occasionally 

 considerably retarded, but the larvae do not acquire sexual 

 organs. 



Gill-rakers, analogous to the gill-rakers of fishes, are found on the 

 branchial arches of many forms. They consist of processes containing 

 a peculiar form of connective tissue. In the larva of the salamander and 

 in the tadpole of the frog there is one row of them on the first and fourth 

 arch and two on the second and third. In the tadpole they are especially 

 well developed and form a filtering apparatus which detains even the 

 smallest particle. 



In the Urodela the gills are not covered by an operculum, 

 but a slight cutaneous fold from the hyoid arch represents this 

 structure. All Amphibia with the exception of the lungless 

 forms referred to on p. 278 possess lungs. These are simple 

 sacs with more or less sacculated walls ; in the Anura they open 

 either directly or by short bronchi {Aglossa) into a laryngo- 

 tracheal chamber which communicates by the slit-like glottis 

 with the pharynx. The laryngo-tracheal chamber is supported 

 by cartilages which can be identified with some of those found 

 in the larynx of the higher forms, and in the Anura contains 

 vocal chords. In the Gym7iophiona and some of the more 

 elongated Urodeles {Siren, Amphiuma, Cryptdbranchus) there is 

 a median tracheal tube supported by cartilages. Respiration 

 is effected by a swallowing movement, the air being drawn into 

 the mouth through the nares with the mouth closed. The 



