158 



ANIMAL FORMS 



are better developed, but of the same structure as in the 

 lancelet. There is no bony matter in the skeleton, and 

 there are no scales. The nasal opening is single on the top 

 of the front of the head. 



There are about twenty-five species in this class. Some 

 of them, called lampreys, ascend the streams from the sea 



FIG. 99. Lampreys. 



in the spring for the purpose of spawning. The young 

 undergo a metamorphosis, at first being blind and tooth- 

 less. The adults feed mostly on the blood of fishes, which 

 they suck after scraping a hole in the flesh with their rasp- 

 like teeth. The others, called hag-fishes, live in the sea 

 and bore into the bodies of other fishes, whose muscles they 

 devour. All are slender, smooth, and eel-shaped. 



From their structure and a few fossil remains we sup- 

 pose that these eel-like forms existed long ago, probably be- 

 fore the more highly developed sharks and bony fishes made 

 their appearance, but it is difficult to determine whether 

 their simple organization is of such long standing or is not 

 in part the result of semiparasitic habits, or a life spent 



