70 



H. L. Bruner 



and Fischer (1864); tbe participation of the mylohyoid in closing 

 the gill openings was pointed out by Fischer. 



Accessory structures which aid in closing and protecting the gill 

 clefts are the soft rakers at their inner margins and certain folds 

 or membranes on the outside. The simplest conditions, though pro- 

 bably not the most primitive, occur in the lower urodeles, in which 

 the gill clefts open directly to the exterior and not into an opercular 

 Chamber. In Cryptobranchus and Amphiuma the Single cleft on each 

 side of the neck is guarded by two fleshy ups, one behind and one 



Fig. 2. 



Triton alpestris. Transverse section of an 18 mm. larva. I and // gill clefts, the second closed 



below by the uuited branchial membranes; oc opercular Chamber; sm posterior part of submaxillaris 



muscle in the opercnlar membrane. 



in front of the opening. Gill rakers are entirely wanting in Amphiuma\ 

 in Cryptobranchus they are reduced to rudimentary papillae which 

 are more or less irregulär in their arrangement. In Necturus the 

 two gill clefts on each side are surrounded at their inner margins 

 by short blunt papillae, from six to twelve in number, and so arranged 

 that those on one side of the opening alternate with those on the 

 other side. Externally a fleshy lip protects the anterior margin of 

 the first cleft and a similar fold lies behind the posterior cleft. A 

 Single fold between the two clefts furnishes protection to both. The 

 three gill clefts of Siren are provided with conspicuous gill rakers 

 similar in form to those of certain fishes, but composed entirely of 

 soft tissues. The external folds are similar to those of Necturus, 



