VERTEBRATA. 251 



are feeding upon. The Hag-fislies are parasitic upon 

 living fish, and the Lampreys eat chiefly dead fish or 

 other carrion. It will be remembered that Roman 

 epicures are said to have fed their lampreys now and 

 then upon a drowned slave. The Gyclostomi, unlike 

 other fishes, have no scales. They have the beginnings 

 of what constitutes the skeleton in other animals. 

 The notochord persists, instead of being replaced by 

 the backbone, as in higher types; and it is supple- 

 mented by bars of gristle (cartilage), which represent 

 the beginnings of joints of the backbone. There is 

 a brain, and a cartilage skull surrounding it, which 

 has no bones except in the basal part; and the 

 mouth is armed with teeth. Although in outward 

 appearance these fishes are rather wormlike, it will 

 be seen that they represent an immense advance on 

 Amphioxus in having a brain, teeth, and the be- 

 ginnings of a skeleton. They have, moreover, two 

 eyes and a distinct heart, so that altogether they 

 resemble Amphioxus only in the persistence of the 

 notochord, a peculiarity that they share with other 

 groups of fishes presently to be described. Their 

 whole structure, and especially that of the head, is so 

 greatly modified in accordance with their suctorial 

 habits, that it is difiicult to draw from the study of 

 these animals any conclusions as to the way in which 

 they have been developed from lower forms ; but it 

 is a suggestive fact that one of the Lampreys has a 

 larval form, with no eyes and no teeth, which acquires 

 both as it grows older. This larval form was at first 



