344 MANUAL OF ZOOLOGY. 



wards seen, there generally exists between the head and the 

 scapular arch a great cavity or gap on each side, within which 

 are contained the branchiae. The cavity thus formed opens 

 externally on each side of the neck by a single vertical fissure 

 or " gill-slit," closed by a broad flap, called the " gill-cover " 

 or " operculum," and by a membrane termed the " branchi- 

 ostegal membrane." 



The gill-cover (fig. 126, /, 0, s, i) is composed of a chain of 

 broad flat bones, termed the opercular bones. Of these, the 

 innermost articulates with the skull (tympano-mandibular arch), 

 and is called the " praa-operculum ; " the next is a large bone 

 called the "operculum" proper; and the remaining two bones, 

 called respectively the "sub-operculum" and "inter-operculum," 

 form, with the operculum proper, the edge of the gill-cover. 

 These various bones are united together by membrane, and 

 they form collectively a kind of movable door, by means of 

 which the branchial chamber can be alternately opened and 

 shut. Besides the gill-cover, however, the branchial chamber 

 is closed by a membrane called the " branchiostegal mem- 

 brane," which is attached to the os hyoides. The membrane 

 is supported and spread out by a number of slender curved 

 spines, which are attached to the lateral branches of the 

 hyoid bone, act very much as the ribs of an umbrella, and are 

 known as the "branchiostegal rays" (fig. 126, d). 



The hyoid arch of fishes is attached to the temporal bones 

 of the skull by means of two slender styliform bones, which 

 correspond to the styloid processes of man, and are called the 

 " stylohyal " bones (fig. I27,/). The rest of the hyoid arch is 

 composed of a central portion and two lateral branches. Each 

 branch is composed of the following parts : i. A triangular 

 bone attached above to the stylohyal, and termed the " epihyal 

 bone" (fig. 127, e) ; 2. A much longer bone, known as the 

 " ceratohyal " (d). The central portion of the hyoid arch is 

 made up of two small polyhedral bones the " basihyals " (b). 

 From the basihyal there extends forwards in many fishes a 

 slender bone, which supports the tongue, and is termed the 

 " glossohyal " or " lingual " bone (a). There is also another 

 compressed bone, which extends backwards from the basihyals, 

 and which is known as the " urohyal bone " (c). This last- 

 mentioned bone is of importance, as it often extends back- 

 wards to the point of union of the coracoid bones, and thus 

 forms the isthmus which separates the two branchial apertures. 



From the outer margins of the epihyal and ceratohyal bones 

 on each side arise the slender curved " branchiostegal rays," 

 which have been previously mentioned. There are usually 



