The Mechanism of Pulmonary Respiration in Amphibians with Gill Clefts. 81 



fold which completely surrounds the opening. Its anterior half is 

 muck thickened and more or less rigid and thus forms a support 

 for the posterior half which is thin and flexible and acts as a valve. 



Summary and Comment. 



1. The mechanism of pulmonary respiration in amphibians with gill 

 clefts is a force-pump mechanism similar to that of higher amphibians. 



2. Definite arrangements exist in all forms for the control of 

 the respiratory media, including apparatus for closing the mouth, 

 gill clefts and nasal passages. 



3. The simplest and most primitive form of this respiratory 

 mechanism is found in Necturus, Proteus and the larvae of higher 

 lungbearing urodeles, a common characteristic of which is a meck- 

 anical choanal valve which prevents the escape of the respiratory 

 media from the mouth through the nasal cavity. In all of these 

 forms respiratory air is taken by the open mouth. 



4. In Siren the respiratory mechanism is similar to that of Nec- 

 turus but somewhat more complicated. Two choanal valves are 

 present, a median valve corresponding to that of Necturus, and a 

 posterior valve which encloses the antorbital cartilage and is moved 

 by a small muscle. By means of the posterior valve the choana 

 can be opened and expiration through the nasal cavity is possible. 



5. Amphiuma is provided with smooth muscles for opening and 

 closing the nasal vestibule and with a complicated choanal mechan- 

 ism, which includes the antorbital cartilage and two striated muscles. 

 By means of these muscles the choana can be opened and closed. 

 Kespiratory air is taken through the nasal cavity, as in higher urodeles. 



6. In Cryptobranchus the choana is closed by the hyoid arch. 

 Respiration, both bucco-pharyngeal and pulmonary, is similar to that 

 of Amphiuma. 



7. The character of the respiratory mechanism of the Perenni- 

 branchiata seems to indicate a) that Necturus and Proteus are slightly 

 modified permanent larvae, b) that Siren is a modified larva whose 

 choanal mechanism has been brought under muscular control. 



8. The Cryptobranchiata appear to be much modified permanent 

 larvae whose development has been arrested during the meta- 

 morphosis. They have retained the essential parts of the acquatic 

 respiratory mechanism excepting the gills, while at the same time 

 they have the pulmonary mechanism of the higher urodeles. 



Horpuolog. Jahrbuch. 48. Q 



