432 ZOOLOGY. 



available for western students who have not access to speci- 

 mens of the dinner. 



The perch has the general form of a flattened spindle, for 

 it tapers down at either end and is compressed laterally. 

 There is no neck marked oif externally, and the head ap- 

 pears as the direct continuation of the body, but separated 

 from it by a fissure on either side ; this is the opening of 

 the gills, which extends from above downwards and curves 

 forward, nearly meeting its fellow on the median line of the 

 under jaw ; upon opening the gill-slit the pectinate or comb- 

 like gills or branchiae are seen within. There are four sets 

 of branchial filaments, each set attached to a separate de- 

 scending arch, in front of each of which is a slit leading into 

 the cavity of the mouth ; but there is no slit behind the 

 last gill. The branchiae are protected externally by the gill- 

 cover or operculum, which is attached in front, but is free 

 behind, where it forms the front edge of the gill-slit ; it is 

 composed of four distinct parts : 1. The praeoperculum 

 nearest the eye, and with its lowest corner almost a right 

 angle; its posterior and vertical edge is furnished with 

 numerous minute projecting spines. 2. Appended to the 

 underside of the margin thus armed is the operculum. 3. 

 Below the praeoperculum is the interoperculum, which par- 

 tially covers up 4, the suboperculum. Each of these parts 

 has a separate bony support ; all four bones are developed 

 only in the Teleosts ; in sturgeons, for example, there is 

 only an operculum, to which in other Ganoids other parts 

 are added ; in Selachians the whole apparatus remains 

 undeveloped. 



The mouth is placed in front ; the upper lip is capable of 

 independent motion, being supported by the praemaxillary 

 bones, which are but loosely attached to the cranium, though 

 in many other fishes the union is closer. The eyes are large 

 and lidlcss ; just in front of each eye is an opening of the 

 size of a pin's head ; these openings lead into the nasal sacs, 

 of which there are two, but both are without communica- 

 tion with the mouth ; in higher vertebrates, from the Dip- 

 noi upwards and in Myxine, there is such a communication. 

 In the Marsipobranchii there is but a single median nasal sae. 



