AQUATIC VERTEBRATES 367 



parasitic lamprey. The hole which is made in the fish is so large and 

 deep that the fish usually dies as a result of this attack. In the Great 

 Lakes the lampreys have destroyed so many fish that they are believed 

 to be largely responsible for the decline of commercial fishing in that 

 region. 



A lamprey has seven pairs of gill openings, which are more than is 

 found in members of other groups of vertebrates. In many ways the 

 lamprey is either primitive or degenerate — it has no paired appendages, 

 there is only one nostril and that is on the top of the head, and there is 

 no stomach, pancreas, or spleen. If we cut into the body of the lamprey 

 we find that there is no bony skeleton, but there is a large notochord 

 which runs the length of the body and serves to help support the softer 

 body tissues. Also there is a basket composed of cartilage which sup- 

 ports the gills, and there are small pieces of cartilage in the head. 

 When ready to reproduce, lampreys swim up into small, clear streams 

 and build nests by moving rocks with their sucking mouths. The fe- 

 males then lay their eggs in the nest, and the males release sperms over 

 them. Many species of lampreys die soon after reproduction, and it is 

 common to see dozens of dead lampreys washed up on the banks of a 

 stream or floating down with the current shortly after the reproductive 

 season. 



An tgg hatches into a small fish-like creature that looks very different 

 from an adult. In fact, it is so different that it was once classified as a 

 different animal under the name, Ammocoetes. This larval stage resem- 

 bles Amphioxus in many ways and was for a time grouped with it in the 

 Cephalochordata. Then it was discovered that it underwent a metamor- 

 phosis after several years and became a lamprey. This resemblance indi- 

 cates that the lamprey and Amphioxus are probably closely related through 

 a common ancestor. 



Cartilaginous Fishes — Class Chondrichthyes 



The members of this class have a much better developed skeleton 

 than that of the cyclostomes, but the skeleton is composed of cartilage 

 — there is no true bone. Cartilage is a material which is flexible to a 

 certain degree, but is rigid enough to maintain its shape and support 

 softer body parts. The ear flap of man which projects on either side 

 of the head is composed of cartilage covered with skin. We can bend 

 our ears in various directions, but they go back to their original shape 

 when we release the pressure. 



The best known member of this class is the shark and we will use 

 it as an example of the group. The shark has a body which shows a 



