FISHES. 3 1 1 



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

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

 lum," 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 known as 

 the " branchiostegal rays " (fig. 276, d). 



The limSs of fishes depart considerably from the typical form 

 exhibited in the higher Vertebrates. One or both pairs of 

 limbs may be wanting, but when present the limbs are always 

 in the form of fins that is, of expansions of the integument 

 strengthened by bony or cartilaginous fin-rays. The anterior 

 limbs are known as the pectoral fins, and the posterior as the 

 ventral fins ; and they are at once distinguished from the 

 so-called "median" fins by being always disposed in pairs, 

 usually symmetrically. Hence they are often spoken of as the 

 paired fins. 



The fore-limbs or pectoral fins possess in a modified form 

 most of the bones which are present in the anterior extremities 

 of the higher Vertebrata. They vary much in size and in other 

 characters. Sometimes they are enormously expanded, as in 

 the Flying-fish (Exoccetus) ; and at other times they form merely 

 a pair of paddles, as in the extinct Pterichthys. The hind- 

 limbs or ventral fins are wanting in many fishes, and they are 

 less developed and less fixed in position than are the pectorals. 

 In some cases the ventral fins are " abdominal " in position, 

 and are placed more or less towards the hinder part of the 

 body (as in the Sharks, Ganoids, and Mud-fishes). In other 

 cases, they are "thoracic," that is, they are placed beneath the 

 pectorals ; and in some cases they are situated on the sides of 

 the neck in advance of the pectorals, when they are said to be 

 "jugular." In these cases, the pelvic arch is attached to the 

 pectoral arch, and is therefore wholly re\noved from its normal 

 position. 



In addition to the pectoral and ventral fins the homologues 

 of the limbs which may be wanting, fishes are furnished 

 with certain other expansions of the integument, which are 

 " median " in position, and must on no account be confounded 

 with the true " paired " fins. These median fins are variable 



