136 



PISCES. 



of the praeoperculum constitute the branchial operculum, and are 



distinguished as operculum, suboperculum, and interoperculuni. A 

 bone extending from the metapterygoid and quadrate to the upper 

 jaw corresponds to the pterygoid, and is, as a rule, formed of an 

 external (ectopterygoid) and an internal piece (endopterygoid). Then 

 come the palatine bone and the apparatus of the upper jaw, with 

 the prsemaxilla (intermaxilla), which is placed at the front of the 

 snout and is usually moveable, and the very variable, usually tooth- 

 less maxilla. The two limbs of the lower jaw are only rarely fused 

 together in the middle line, and are divided at least into a posterior 



FIG-. 587. Hyoid apparatus and branchial arches of Pereajluvialilis (regne animal). I, hyoid 

 apparatus ; II V, branchial arches ; a, I, c, d, joints of the branchial arches, the upper 

 joints (Ops) are the superior pharyngeal bones (pharyngo-branchials) ; I"/, (Opi) the 

 inferior pharyngeal bones (reduced 5th branchial) ; Cop, copula? ; Eb, branchiostegal 

 rays. 



OS articulare and an anterior dentary ; there may, however, also be 

 an angulare and an operculare. 



Behind the mandibular arch there follows a system of equivalent 

 arches surrounding the pharyngeal cavity. Of these the anterior 

 the hyoid arch bears on its outer edge a number of cartilaginous 

 rods, which serve to support the opercular membrane and are called 

 the branchiostegal rays (fig. 587, Rl>], while the remaining arches are 

 the branchial arches and serve for the support of the branchial 

 lamella? (fig. 587). In the Teleosteans four (seldom three) arches 

 bear gills, while the posterior arch is reduced so that only its ventral 



