194 THE STORY OF FISH LIFE. 



speaking we are right in our determination that 

 this or that particular creature is a fish. But 

 there are many pitfalls, for not a few animals, 

 not even remotely related, are from their general 

 contour classed by the uninitiated as "fish." 

 Those who are on their guard and are familiar 

 with the credentials of fishhood, when suspicion 

 is aroused look for the characteristic scales, fins 

 and gill-openings. Generally all these will be 

 found, but scales may be wanting, so also may 

 the paired fins, but the median fins and gills, 

 never. And so it would seem then that it is 

 easy after all to determine what is a fish. By 

 no means, for the early tadpole stages of the 

 common frog are practically fish, whilst the 

 greatest experts of the year of grace 1901 differ 

 among themselves as to the claims to fishhood 

 which have been put forward on behalf of certain 

 living and fossil forms which we shall now 

 describe. 



Let us take the living forms first. These are 

 represented by the somewhat unfamiliar eel-like 

 lampreys and hag-fishes. In a number of char- 

 acters these differ markedly from the forms 

 hitherto discussed. There are no movable 

 jaws ; there is but a single nostril placed in the 

 middle of the snout ; the mouth is a circular 

 cup-shaped cavity armed with numerous horny 

 teeth ; there are no limbs, no ribs, no gill-arches. 

 The skeleton of the head is cartilaginous ; the 

 vertebral column is represented by an elastic and 

 fibrous rod. The gills are of a quite peculiar 

 pouch-like form, hence the scientific name of the 

 group Marsipobranchii. The skin in the region 



