FISHES AND FISHERIES 239 



along the Atlantic seacoast. They are slender, translucent 

 little creatures of very lowly organization. 



The class Cyclostomata (Gr. kyklos, circle; stoma, mouth) 

 includes the lampreys and hag-fish, slender, eel-like forms hav- 

 ing a sucker-like mouth but no jaws. The lampreys, genus 

 Petromyzon, occur in both fresh and salt water, those living 

 in the sea ascending rivers to spawn. They sometimes attach 

 themselves to fishes by their sucker-like mouth and rasp the 

 skin and suck the blood. In this way they are of economic 

 importance as they may thus destroy some of the food fishes. 

 The hag-fish may burrow into the abdominal cavity of other 

 fish and devour the entire flesh and viscera in a short time. 



The other five classes are the familiar ones of the Pisces 

 (L. piscis, fish), or fishes, Amphibia (Gr. amphi, on both sides; 

 bios, life), or frogs, toads and salamanders, Reptilia (L. repo, 



FIG. 112. A lamprey, Petromyzon marinus. (After Goode.) 



to creep), or turtles, snakes and crocodiles, AvesCL. avis, bird), 

 or birds, and Mammalia (L. mamma, breast), or mammals. 

 The fishes constitute the largest class of vertebrate animals, 

 about 13,000 species being known, 3000 of which live in North 

 America. They occur in almost all ponds, lakes and streams 

 and in the ocean, and vary in size from the great basking shark, 

 Cetorhinus, which reaches a length of thirty-six feet, to the dwarf 

 goby, Mistichthys, which is less than half an inch long. Be- 

 tween these extremes is found every variety in size, form 

 and relative proportions. The body may be greatly elongated 

 and almost cylindrical as in the eels, or long and flattened 

 from side to side as in the ribbon-fishes, or globe-shaped as 

 in the globe-fish. 



