FISHES OF MINNESOTA 5 



of the opercle is contained in the length of the fish. Depth in length 

 means the number of times the greatest distance from the back-, or 

 dorsal side, to the belly, or ventral side, is contained in the length 

 of the fish. Ei/e in head, the number of times the width of the 

 entire eye is contained in the head, measuring from the tip of the 

 snout to the posterior edge of the opercle. Eye in snout, the number 

 of times the width of the eye is contained in the length of the snout, 

 measuring from the anterior edge of the eye to the tip of the snout. 

 Occasionally the length of spines, length of fins and also other parts 

 are Compared with the total length and with other measurements. 

 The above expressions are often abbreviated as "Head 4," which 

 means that the head is contained four times in the length of the 

 fish. A fish is said to be compressed when flattened laterally, as is 

 the case with the white-fish and even the perch, and depressed when 

 flattened from above, or dorso-vmtralVy. The head of the cat-fish is 

 much depressed. 



The heads of the various species vary greatly in other respects 

 than those already considered. There are always four nostrilis, 

 the posterior and anterior on each side, being near each other or 

 well separated. In the cat-fishes the posterior nostril is accom- 

 panied by a barbel. In addition to the nasal and maxillary barbels 

 there may be barbels on the lower jaw. The snout in some fishes 

 is long and pointed, while in others it is blunt and rounded. It is 

 necessary to study the snouts in identifying minnows. 



Besides the teeth in the upper and lower jaws already referred 

 to, some fishes have teeth on the vomer, a bone located under the 

 nasal openings in the front of the roof of the mouth, on the palatine 

 hones, which are located just back of the vomer, on the pterygoids, 

 which are located back of the palatines, and some on the tongue 

 itself. Teeth located in any or all of the above named places are 

 known as the ordinary teeth. In some fishes they are present on 

 all these bones, while in others on only a portion of them, and in 

 many there are no ordinary teeth at all, viz., the suckers and min- 

 •nows. The perch has teeth on the upper and lower jaws and the 

 vomer. 



Fishes breathe by means of gills, the perch having four pairs 

 borne on the curved bones known .as the gill-arches. Each gill is 

 composed of numerous fringe-like filaments through which the 

 blood circulates and comes in contact with the oxygen absorbed 

 in the water. The openings between the gills are known as the 

 gill-slits. The one found behind the fourth gill in the perch is ab- 

 sent in some fishes. On the front and sides of the gill arches are the 



