LOWEST CLASSES OF VERTEBRATES 



in a skull. But the skull is imperfectly developed and with- 

 out jaws; there is no shoulder girdle, no pelvis, no limbs, no 

 ribs and no paired fins. There is a single median nostril, 

 the gills are purse-shaped, the skin is naked like that of an 

 eel and the skeleton is cartilaginous. The gills are in the 

 form of a fixed sac, the gill openings consist of a row of tiny 

 round holes along the side of the body, and the mouth is 

 specially formed for suction. 



It is evident from the foregoing characters that the lam- 

 preys are creatures of very simple form, lacking almost all 

 the evidences of special development which characterize the 

 higher fishes. Externally, their very modest median fins are 

 the only visible signs that they are not marine worms. 



THE SEA LAMPREY 1 is the best and most available example 

 of the Class Mar-si-po-branch'ii. 'The mouth is completely 

 circular, and forms a great and powerful sucker, surrounded 

 by fleshy lips that are supported on a framework of cartilage 

 and studded with tentacles. This mouth is covered over its 

 entire interior surface with strong teeth arranged in concen- 

 tric circles. A large double tooth, situated above the aperture 

 of the mouth, indicates the situation of the upper jaw, and 

 seven or eight great teeth represent the lower jaw. Even 

 the tongue carries three large teeth, deeply serrated on their 

 edge." 2 



With a mouth specially formed and savagely equipped 

 for suction, it is no surprise to find that this creature is a 

 blood-sucking parasite, preying upon other forms of marine 



1 Pet-ro-my'zon ma-ri'nus. 



2 "Fishery Industries of the United States," Part I, p. 677. 



