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 
Fie. 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 
. 
