Chap. 33 



VERTEBRATES LOWER CHORDATES AND FISHES 



INVERTEBRATE 



667 



VERTEBRATE 



Fig. 33.4. Diagrams to show the difference in body plan between an invertebrate 

 (an annelid worm) and a vertebrate. In the latter, the inner ends of the vertebrae 

 (centra) are in the area occupied by the notochord in the embryo. 



lamprey that became land-locked in ancient times. Lampreys are abundant in 

 the Finger Lakes of New York and tributary streams and it is estimated that 

 tons of fish are killed by them every year. Before the breeding season, when 

 lampreys are hungriest, the upturned body of a dead fish with its quota of 

 lamprey holes is a common sight in Cayuga and the other Finger Lakes. 



In spring, these lampreys go up the creeks to make their nests and breed. 

 Like many fishes, they clear the bottom of gravel by fanning with their tails. 

 They pick up stones with their sucking mouths, which during this season are 

 turned from blood-sucking to domestic work. 



Fishes 



Fishes are the dominant aquatic animals, more numerous than any other 

 vertebrates except birds. Various kinds live in fresh, brackish and salt water 

 — in clean water and on mucky bottoms. Some stay near the surface, others 

 live at great depths where there is no light except from the light organs of 



Fig. 33.5. Lake lamprey eel, Petromyzon marinus, attached to a fish. Above 

 the pectoral fin is a scar where another lamprey made a ragged opening with its 

 rasping tongue. 



