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MARINE PRODUCTS OF COMMERCE 



have no vertebrae, as their "backbone" consists of a continuous cartilaginous rod. 

 Neither do they have jaws, the mouth being a mere lengthwise slit. The median 

 fins, that is, the dorsal, caudal, and anal, are composed of continuous membranes 

 without rays. Paired fins, or limbs, are entirely missing. 



The next higher group of fishlike creatures are the lampreys and hagfish (Fig. 

 10-1). These animals are known to the scientist as the Cyclostomi. Although they, 

 too, are primitive forms they do represent a forward step in development from 

 the lancelets. They are eel-like or wormlike in shape and have a fairly well- 



{C curtesy U. S. Fish and Wildlife Service) 



Fig. 10-1. The great sea lamprey. (Petromyzon rmirinus.) 



developed, though continuous, spinal column. They have an imperfectly de- 

 veloped skull, heart, and brain. They still have no jaws and no paired fins repre- 

 senting the limbs of the higher vertebrates. The mouth is round or oval, forming 

 a suctorial disk, and is generally provided with rasping teeth, which are their only 

 hard (bony) parts, the skeleton being cartilaginous as in the lancelets. 



Cyclostomes attach themselves with their suctorial mouths to their hosts, gen- 

 erally larger fishes, and eventually bore with their teeth into the abdominal cavity 

 or vital parts, causing the hosts to die. In some areas the lampreys are highly 

 destructive of other fish. However, some of them reach a length of about 3 feet 

 and become food fishes of shght importance. 



The primitive forms already mentioned have not been found in the rocks 

 probably because they had no real bones or spines, and, therefore, no part of them 

 became fossilized. 



The next higher group of animals are the sharks and skates and rays. Although 

 their skeletons, too, are cartilaginous, they have a spinal column composed of 

 vertebrae. They have jaws, though the upper one is attached to the skull, defi- 

 nitely developed paired fins, and well-developed gills. Therefore, they meet all 

 the requirements of the definition of a fish offered near the beginning of this 

 section. 



