The Skeleton of the Fish 45 



as the opercle (25). Below it, joined by a suture, is the sub- 

 opercle (26). Before it is the prominent ridge of the preopercle 

 (24), which curves forward below and forms a more or less 

 distinct angle, often armed with serrations or spines. In some 

 cases this armature is very highly developed. The interopercle 

 (27) lies below the preopercle and parallel with the lower limb. 



Branchial Bones. The bones of the branchial apparatus or 

 gills are very numerous and complex, as well as subject to im- 

 portant variations. In many fishes some of these bones are co- 

 ossified, and in other cases some are wanting. The tongue may 

 be considered as belonging to this series, as the bones of the 

 gills are attached to its axis below. 



In the striped bass,, as in most fishes, the tongue, gristly and 

 immovable, is formed anteriorly by a bone called the glossohyal 

 (37). Behind this are the basihyals (36), and still farther back, 

 on the side, is the ceratohyal (35). To the basihyals is attached 

 a bone extending downward and free behind the urohyal (38). 

 Behind the ceratohyal and continuous with it is the epihyal (34), 

 to which behind is attached the narrow interhyal (33). On the 

 under surface of the ceratohyal and the epihyal are attached 

 the branchiostegals (39). These are slender rays supporting a 

 membrane beneath the gills, seven in number on each side in the 

 striped bass, but much more numerous in some groups of fishes. 

 The gill membranes connecting the branchiostegals are in the 

 striped bass entirely separate from each other. In other fishes 

 they may be broadly joined across the fleshy interspace between 

 the gill-openings, known as the isthmus, or again they may be 

 grown fast to the isthmus itself, so that the gill-openings of the 

 two sides are widely separated. 



The Gill-arches. The gills are attached to four bony arches 

 with a fifth of the same nature, but totally modified by the 

 presence of teeth, and very rarely having on it any of the gill- 

 fringes. The fifth arch thus modified to serve in mastication 

 instead of respiration is known collectively as the lower pharyn- 

 geals (46). Opposite these are the upper pharyngeals (45). 



The gill-arches are suspended to the cranium from above by 

 the suspensory pharyngeal (44). Each arch contains three parts 

 the epibranchial (43), above, the ceratobranchial (42), forming 

 the middle part, and the hypobranchial (41), the lower part 



