CHAPTER XXII 

 SUBPHYLUM VERTEBRATA, CLASS 3, PISCES 



In true fishes the chorda is usually replaced by bony 

 vertebrae, the gills are protected by an operculum, or " gill- 

 cover/' and the body is covered with scales which are never 

 of the placoid type, as in sharks, but consist of little bony 

 plates which overlap like shingles. In general form and 

 position^the fins are much like those of sharks, but are often 

 supported by bony rays. There are two subclasses of 

 fishes : 



Subclass 1. Teleostomi. Fishes with a skeleton composed largely 

 of bone; with gills, but usually without lungs. 



Subclass 2. Dipnoi. Fishes with a skeleton made up of cartilage 

 and bone, and with a single or paired lung. 



As an example of the subclass Teleostomi, the yellow 

 perch, which is one of the commonest and most widely 

 distributed fishes in North America, will be considered. 



THE YELLOW PERCH, Perca Flavescens Mitchill. 



Self-maintenance. The perch (Fig. 94) is versatile in 

 every way, and this is particularly true of its food habits. 

 It may feed on microscopic animals in the water such as 

 Daphnia or Cyclops (Fig. 31), or eat snails, insect larvae, 

 aquatic insects, minnows, fish eggs, and even plants. The 

 minute animals are secured by straining the water which 

 is continually entering the mouth for respiratory purposes 

 and passing out behind the operculum. This straining is 

 accomplished by a series of minute finger-like projections, 

 the gill-rakers, attached along the inner margins of the 

 gill-arches and spread across the spaces where water leaves 



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