76 



H. L. Bruner 



~ pc 



Its fibres run obliquely forward and laterad and insert on the posteic- 

 lateral angle of the antorbital cartilage. By the contraction of this 

 muscle the anterior end of the cartilage is moved laterad and the 



choana beconies a large trian- 

 gulär opening which can not be 

 covered by the median valve. 

 The choana is closed by 

 the Joint action of the median 

 and posterior valves. When the 

 m. posterior choanae relaxes, 

 the elasticity of the adjacent 

 tissues causes a return move- 

 ment of the cartilage and valve, 

 but the return is probably ac- 

 celerated by the adductor muscles 

 of the mandible which lie late- 

 ral and posterior from the 

 choana. External pressure in 

 this region in a recently killed 

 specimen causes prompt closing 

 of the opening, and siuce the 

 adductor muscles contract to 

 close the mouth during the act 

 of inspiration, they probably press on the posterior valve at this 

 time. When the valve returns to its median position the choana 

 is easily covered and protected by the median valve. 



The character of the posterior choanal valve of Siren makes 

 possible the expulsion of water from the mouth through the nasal 

 cavity and explains the currents which occasionally stream from the 

 nostrils during branchial respiration. 



Siren lacirtina. Choanal region from above (some- 



ivhat diagrammatic). ac antorbital cartilage; 



n choana; os orbito-sphenoid; pc m. posterior 



choanae; tr trabecula cranii. 



The Choanal Mechanism of Amphiuma. 



A valve-like fold at the choanal opening of Amphiuma is menti- 

 oned in a casual way by Cüvier (1827, p. 5) and Wagler (1830, 

 p. 314) whose observations were accepted, apparently without further 

 examination, by Fischer (1864, p. 114). The description of the latter 

 is as follows: »Vor jeder Öffnung liegt eine Falte der Gaumenhaut, 

 die sich leicht nach hinten so über die Öffnurjg legt, daß letztere 

 vollständig geschlossen wird.« A similar Statement is made by Cope 

 (1889, p. 216). 



