FISHES. 113 



each, but really consisting of two halves firmly united along 

 the middle line. The " soft rays " are composed of several 

 slender spines proceeding from a common base, and each 

 divided transversely into numerous short pieces. The soft 

 rays occur in many fishes in different fins, but they are in- 

 variably found in the caudal fin or tail (fig. 487, c). The 

 rays of the median fins, -whatever their character may be, 

 alvrays articulate by a hinge-joint with the heads of the in- 

 terspinous bones. 



The sJciill 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 oper- 

 culum. For reasons connected with the respiratory process 

 in fishes, 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 ca^dty 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. 488, 7^, 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-mandibu- 

 lar arch), and is called the " pra?-operculum ; " the next is a 

 large bone called the " operculum " proper ; and the remain- 

 ing 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 to- 

 gether 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 membrane," which is attached to 

 the OS hyoides. This 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 known as the " branchiostegal rays " 

 (fig. 488, d). In many Ganoid fishes the place of the 



VOL. II. H 



