160 Descriptive Zoology. 



The Protection of the Gills. The gills are really ex- 

 ternal organs. From the nature of their work they must 

 have very thin external coatings, and so are correspond- 

 ingly delicate. Hence the strong yet flexible gill cover. 

 The more technical name for this is the opercle. It con- 

 sists of several parts, the opercle proper, subopercle, 

 preopercle, and interopercle (see Fig. 100). Overlapping 

 from front to back, and being under muscular control so 

 that they can be held down with considerable force when 

 necessary, they constitute a very good shield. In addition 

 to the opercle there is a gill cover below, called -the bran- 

 chiostegal membrane. It is a tough, yet thin, mem- 

 brane supported by several small curved bones, the 

 branchiostegal rays. A fish carries about its head organs 

 that are of vital importance and of most delicate texture, 

 yet it dashes among more or less rough aquatic plants and 

 after fishes that are well armed with spines. It is safe in 

 doing this because of the double set of gill covers, one soft 

 and one bony. 



When the perch swallows a spiny fish, still struggling, 

 will not the soft gills be torn from the inside, producing 

 serious injury ? The use of the bony, toothlike gill rakers 

 projecting on the inner surface of the gill arches is now 

 apparent. The gill rakers also serve as a strainer in 

 swallowing smaller particles of food, and some authorities 

 say that the gill rakers serve, to a certain extent, as teeth in 

 crushing the food. 



When the fish seizes its prey, it of course takes water 

 into the mouth with it; but this is allowed to pass out 

 through the gill openings, and probably only a little is swal- 

 lowed with the food. 



The Sense Organs of the Perch. The perch has well- 

 developed eyes, but without movable lids. If any one 



