464 MANUAL OF ZOOLOGY. 



These form a series of dagger-shaped bones, plunged in the 

 middle line of the body between the great lateral muscles 

 which make up the greater part of the body of a fish. The 

 internal ends or points of the interspinous bones are attached 

 by ligament to the spinous processes of the vertebrae ; whilst 

 to their outer ends are articulated the " rays " of the so-called 

 " median " fins, which will be hereafter described. As a rule, 

 there is only one interspinous bone to each spinous process, 

 but in the Flat-fishes (Sole, Turbot, &c.) there are two. 



Besides the fins which represent the limbs (pectoral and 

 ventral fins), fishes possess other fins placed in the middle line 

 of the body, and all of these alike are supported by bony spines 

 or " rays," which are of two kinds, termed respectively " spinous 

 rays" and " soft rays." The " spinous rays" are simple bony 

 spines, apparently composed of a single piece each, but really 

 consisting of two halves firmly united along the middle line. 

 The " soft rays " are composed of several slender spines pro- 

 ceeding from a common base, and all divided transversely into 

 numerous short pieces. The soft rays occur in many fishes in 

 different fins, but they are invariably found in the caudal fin or 

 tail (fig. 248, c). The rays of the median fins, whatever their 

 character may be, always articulate by a hinge-joint with the 

 heads of the interspinous bones. 



The skull of the bony fishes is an extremely complicated 

 structure, and it is impossible to enter into its composition 

 here. The only portions of the skull which require special 

 mention are the bones which form the gill-cover or operculum, 

 and the hyoid bone with its appendages. For reasons con- 

 nected with the respiratory process in fishes, as will be after- 

 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 " branchiostegal 

 membrane." 



The gill-cover (fig. 249, p, 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 " prse-operculum ; " 

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

 ing two bones, called respectively the " sub-operculum " and "inter-oper- 

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

 These various bones are united together by membrane, and they form col- 

 lectively a kind of movable door, by means of which the branchial cham- 

 ber can be alternately opened and shut. Besides the gill-cover, however, 

 the branchial chamber is closed by a membrane called the " branchiostegal 



