412 DAVIDSON BLACK 



In urodeles the musculature concerned in the act of inspira- 

 tion is derived both from the hyoid and branchial segments. 

 Thus in Siren the cerato-hyoideus externus, interbranchialis 

 and le^'ator hyoidei muscles which are innervated by the VII 

 nerve, act as inspirators, together with the levators of the bran- 

 chial arches, the cerato-hyoideus internus and branchial con- 

 strictors, innervated by the IX-X nerves. Similarly in Triton 

 the m. quadrato-pectoralis (m. constrictor colli) acts in common 

 with the branchial levators and constrictors, and in Crypto- 

 branchus the m. cephalo-dorso pectoralis is associated in the 

 same way with the branchial inspirators. 



Hyo-branchial skeleton. It is obvious from this brief review 

 that, though the mechanics of respiration may be essentially 

 similar in both anurans and urodeles, yet the units of the mus- 

 culature upon whose action the respiratory movements depend 

 are fundamently different in their arrangement in the two orders. 

 The immediate cause of the muscular arrangement peculiar to 

 anurans is undoubtedly to be found in the characteristic mor- 

 phology of the hyo-branchial skeleton in these forms. Thus 

 the fusion of the hyoid with the broad basilingual plate and 

 the final almost complete obliteration of branchiomeric arrange- 

 ment in the adult anuran cartilago hyoidea has resulted in the 

 specialization of the mm. levatores arcuum branchialium to 

 form the mm. petro-hyoidei, and in the marked reduction in 

 the respiratory importance of the facial musculature. On the 

 other hand, in urodeles the absence of such extensive fusion of 

 the hyoid and branchial arches has favored the retention of 

 the hyoid and branchial levator muscles as individual units, 

 and has even made possible the retention in some cases of a 

 primitive superficial hyoid constrictor musculature (e.g., Triton 

 and Cryptobranchus). It is apparently owing to the retention 

 of this modified branchiomeric arrangement of the visceral 

 musculature in urodeles, even in the absence of gill slits, that 

 the primitive selachian character of the VII-IX-X motor column 

 has not been lost. 



It should be noted here that the peculiar arrangement of the 

 hyo-branchial skeleton becomes early manifest in anuran larvae 



