LOWEST VERTEBRATES. 245 



CHAPTER IX. 



THE LOWEST VERTEBRATES. 



THE Lampreys belong to the very lowest group of 

 vertebrates. They are not regarded as " Fishes " 

 by modern zoologists but present so many peculiarities 

 that they form a sub-class by themselves. In the first place 

 they have no gill-arches such as are found in fishes, but, 

 instead of these, are provided with 6 or 7 gill pouches on each 

 side. They have no ribs or limbs and the skull is im- 

 movably fixed to the backbone ; the skeleton is cartilaginous, 

 and the skin bare of scales. The mouth is circular and 

 adapted for sucking, but when closed is only a slit. It is 

 provided with horny teeth which rasp off the flesh of any fish 

 to which the lamprey attaches itself. Unlike fishes, too, 

 lampreys undergo a metamorphosis and take three or four 

 years to reach maturity. The young are so unlike the adult 

 that they were regarded as distinct animals under the name of 

 Ammoc&Us until their true nature was discovered by watching 

 their metamorphoses. In spite of all these differences 

 Lampreys bear a close resemblance to Eels and have a fin 

 running along the back as in those fishes. 



Sea Lamprey. This is the largest of the three species, 



Petromyzum marinus. reaching to a length of 3 feet, and is 



further distinguished by its colour 



greenish brown or grey mottled with black. In the adult 



state it resides in salt water but always ascends rivers to 



