666 EVOLUTION OF ANIMALS Part V 



them in prominence. On no other group of animals has the human race de- 

 pended so much for food, work, transport, and companionship. Verte- 

 brates have a bewildering capacity for adaptability in form, size, and habit; 

 mouse and whale, ground mole and eagle, flying fish and antelope, flounder 

 on the sea bottom and squirrel on the tree trunk, penguins grand marching on 

 the ice and dancers in the ballroom. Differences in size do not alter the basic 

 pattern. Learn the anatomy of a mouse and you can understand that of an 

 elephant. An elephant's trunk is still a nose. 



Animals in the Subphylum Vertebrata fall into 7 groups: 



Class Cyclostomata. Lamprey eels. 



Class Chondrichthyes. Cartilaginous fishes. Skeleton cartilaginous, dogfish, 



shark. 

 Class Osteichthyes. Bony fishes. Skeleton more or less bony; trout, perch, true 



eels. 

 Class Amphibia. Salamanders, frogs, and toads. Skin moist and glandular; 



gills temporary or permanent, rarely lacking; five-fingered and four-toed 



limbs. 

 Class Reptilia. Turtles, lizards, and snakes. Cold-blooded; embryo developing 



in a sac (amnion); dry skin with outer horny layer of scales. 

 Class Aves. Birds. Feathers. 

 Class Mammalia. Mammals, including man. Hair and milk glands. 



In addition to the three unique characteristics of all chordates the leading 

 ones of the vertebrates are: an internal skeleton of cartilage or bone; a verte- 

 bral column, replacing the notochord of lower chordates; usually two pairs of 

 appendages, fins or jointed limbs; a ventral heart with two or more chambers; 

 a closed circulatory system; a large coelom or body cavity containing essential 

 organs. 



Lamprey Eels 



The lamprey eels are usually a foot or two long with round, sucking mouths 

 without jaws, numerous gill clefts, no paired fins, a poorly developed skull, 

 and no scales in the mucous skin. Lampreys are neither eels nor true fishes, 

 although they have some resemblance to both. Almost every feature of a 

 lamprey eel is peculiar. Its most striking one is the large suction disk that 

 surrounds the mouth and bears circlets of horny teeth upon its surface. The 

 adult lamprey fastens this disk to the side of a fish, rasps the teeth against the 

 flesh and sucks out the blood while the fish carries its rider about as long as it 

 can swim (Fig. 33.5). 



Most species of lampreys pass part of their fives in salt water but some are 

 land-locked in fresh water and all of them breed there. The lake lamprey eel, 

 Petromyzon marinus, is generally considered the same species as the great sea 



