196 BRANCH C'HOHDATA 



slimy and very smooth. The gills of the lampreys open into a 

 respiratory tu})e lying below the gullet. 



F'ig. 157. — Lamprey {Pdromy'zon mari'nus). (After Goode.) 



They are parasitic on fishes and also devour crustaceans. The 

 slimy eels bore into fishes and eat the flesh. 



CLASS II. PIS'CES 



To this class belong the true fishes, common examples of 

 which are the sunfish, perch, salmon, catfish, carp, and trout. 

 They are aquatic, gill-bearing, poikilothermal, usually scaly, 

 bilaterally symmetric finned chordates. 



Shape. — The typical fish is wedge shaped (Fig. 158) at both 

 ends, so that it can pass rapidly through the water. The head 

 is large and pointed, with the viscera situated near it, while the 

 trunk is long and tapering, for the attachment of muscles to flex 

 the tail in locomotion. Usually the body is more or less flat- 

 tened from side to side, though it may be quite cylindric, as in 

 the eel, or flattened dorsoventrally, as in the adult flounder, or 

 the body may be long and slender, as in the pipe-fish and ril)bon- 

 fish. The shape conforms largely to the habits and habitat. 

 The fishes having the under side flattened usually swim near or 

 rest upon the ])ottom at some time, as our catfish, but the broad 

 forms, flattened al:)Ove and below, like the skates and flounders, 

 live upon the Ijottom and are not built for speed. The "flat 

 fishes" in early life have the position common to most fishes, Ijut 

 in adult life the dorsoventral plane becomes horizontal instead 

 of vertical, the eye lies upon the upper side, and the color of the 

 upper side becomes dark like the dorsal side of most fishes, and 

 the under side light like the ventral side. 



