44 SORTATION. 



In the gill region we have the " covers," consisting of the oper- 

 cular with the sub-opercular below it, and the pre-opercular 

 with the inter-operular below it. Under the inter-opercular are 

 the branchiostegals, the rays which protect the gills, and if we 

 were to remove the gill-covers we should see the gill-arches, of 

 which we have a separate outline (Fig. 25). Of these arches there 

 are five, with, as a rule, gills on four of them, the gills being in a 

 cavity below the pharynx, with clefts between the arches through 

 which the water passes from the pharynx on its way out through 

 the gill openings. The three arches in front are each made up of 

 four bones ; in the fourth arch there are but three bones, and the 

 uppermost, when dilated, becomes the upper pharyngeal, the fifth 

 arch, composed of one bone only, becoming the lower pharyngeal. 

 The inner side of the gill-arches, in some cases, carry projections 

 known as gill-rakers, which act as a grating to prevent substances 

 in the water from passing into the gills. The gills are usually held 

 up by two rows of cartilaginous rods along the rounded edge of the 

 arches, but in some fishes there is one row instead of two. The 

 pseudo-branchiae, or false gills, are along the inner side of the gill- 

 covers. They do not assist in respiration except in the fish's early 

 life, and in the adult they receive only arterial blood. 



The gill-arches are bounded by the hyoid arch, which is con- 

 nected with the temporal bones by the stylo-hyoids, its largest 

 member being the ceratohyal, to the inner end of which are 

 attached the branchiostegals, which carry the branchiostegal mem- 

 brane in much the same way as an umbrella is spread by its ribs. 



At the base of the cranial cavity, and in the bony fishes more 

 or less within it, is the ear, a highly-developed organ, consisting 

 essentially of a sac often divided into chambers, each containing an 

 otolith with indentations and grooves to which the acoustic nerve 

 is connected. These otoliths are the bones that people are so fre- 

 quently at a loss to assign a position to. That in the 

 diagram is one of those of a cod (Fig. 26). 



The eye, like the ear, is of different grades of develop- 

 ment. In some fishes it is very highly organised ; in 

 one genus it is in two portions, one for seeing in water 

 and the other for seeing in air, but this fish is not a 

 Britisher. We have no occasion to go into detail ; all 

 we have to deal with is the bony frontier. Below, this 

 Fi 26 - cons ^ sts f tne infraorbitals or suborbitals, of which 

 OTOLITH ^ ne P raeor bital or lachrymal (nearest the mouth) is the 

 OF largest. Above the eye come the frontal and prefrontal, 

 CODFISH. w ^ n tne nasals extending over the anterior half towards 

 the maxillary, while below it, beyond the infraorbitals, 

 an outer ring, whose main elements are the hyomandibular and the 

 quadrate, join on to the mandible by means of the articulary. 



In this hasty run round the perch's head we have by no means 

 mentioned all the bones, but we have said enough to enable us to 

 deal with the identification of the families and genera on the British 

 list. Before, however, venturing on the systematic, we must have a 

 few examples of the way in which our keys are worked, and for this 

 purpose we will give the tabular scheme complete. 



