208 



ANATOMY OF VERTEBRATES. 



former, and becomes through the medium of 27, a retractor of the 

 mandible. When the retractor hyoidei relaxes and the mandible 

 is the fixed point, the genio-hyoidei, fig. 135, 27, become pro- 

 tractors of the hyoid arch. In some fishes a transverse muscle, 

 repeating the characters of 21, fig. 135, passes from one ceratohyal 

 to the other. The branchiostegal appendage has muscles for rais- 

 ino- and depressing, divaricating and approximating the rays. The 

 levator hranchiostegorum, figs. 135 and 136, 28, arises from the 

 inner surface of the hinder half of the opercular bone and from 

 a contiguous part of the subopercular, and is continued from ray 

 to ray to the lowest, being loosely attached to their inner surface. 

 It forms a kind of muscular capside of the branclual chamber. 

 The depressor hranchiostegorum, fig. 135, d, arises from the lower 

 end of the ceratohyal and passes obliquely backward, crossing 

 its fellow, to be inserted into the inferior branchiostegal ray. 

 These muscles regulate the capacity of the branchial chamber, 



137 



and mainly act njion the water it contains : they show accord- 

 ingly much diversity, especially 2s, in relation to the respiratory 

 characteristics and comiected peculiarities in different fishes. In 



