PHYLUM CHORDATA 231 



Class 3. Pisces. Fish-like vertebrates with cartilaginous or bony 

 skeletons, true jaws, scales, and fins supported by rays. True fishes. 



Class 4. Amphibia. Slimy-skinned vertebrates without exoskele- 

 tons; with pentadactyl limbs; no claws on toes; heart three-chambered. 

 The young are aquatic and breathe by means of gills; the adults are 

 usually more or less terrestrial and breathe through lungs. Frogs, toads, 

 salamanders. 



Class 5. Reptilia. Scaly-skinned vertebrates, which breathe by 

 means of lungs and have a three- or four-chambered heart; claws on 

 the toes. Lizards, snakes, turtles, crocodiles, etc. 



Class 6. Aves. Feathered, scaly-skinned vertebrates with four- 

 chambered heart and warm blood, but no teeth. Birds. 



Class 7. Mammalia. Warm-blooded vertebrates with hair and 

 mammary, or milk, glands. Whales, seals, bats, dogs, horses, monkeys, 

 etc. 



CLASS 1. CYCLOSTOMATA 



The cyclostomes lack true jaws and are without paired 

 lateral appendages. The nasal pit is single instead of 

 double as in other vertebrates, and there are from seven to 

 fourteen pairs of gill slits. Two kinds of cyclostomes are 

 living at the present time hagfishes and lampreys. 



The hagfishes are found only on muddy bottoms in the 

 ocean. They bore their way into fishes in order to eat out 

 the soft parts, and are a source of great annoyance to fisher- 

 men because they commonly attack shad, cod, and other 

 fish caught in nets or on hooks. They have a single tooth 

 which they use for boring into their victims. 



Lampreys live both in the ocean and in fresh water. 

 They have a cartilaginous funnel around the mouth which 

 is used as a sucking disc for feeding or attachment. There 

 are numerous teeth inside the mouth and on the tongue. 

 Petromyzon marinus (Fig. 89, B; 91, ^4.) is a common species 

 about the mouths of rivers along the Atlantic coast of the 

 United States. It lives on blood which is obtained by 

 attaching the oral disc to fishes and rasping a hole with 

 the tongue. The larva of this species differs markedly 

 from the adult, and shows several characteristics which 

 resemble those of Amphioxus (Fig. 90, e). Another cyclo- 



