Survey of the Animal Kingdom 331 



central nervous system is dorsal to the digestive tract. All vertebrates have a ven- 

 tral heart with two to four chambers, depending on the species (Figs. 364 and 

 365). 



Class 1 — Cyclostomata (si klo -sto' ma ta) (Gr. kyklos, circle or round; 

 stoma, mouth). — These possess a jawless, circular sucking mouth with a rasping 

 tongue. They are aquatic, fishlike, and have a median unpaired nostril. There 

 are no lateral appendages or fins. They possess no scales and a permanent noto- 

 chord. 



Examples: Hagfishes and lampreys (Fig. 143). 



Class 2 — Elasmobranchii (e las mo -brang' ki i) (Gr. elasmos, plate; branchia, 

 gills). — These types possess vascular gills or branchia supported by cartilaginous, 

 platelike structures known as gill plates. They are fishlike animals with jaws, 

 paired fins, and placoid (platelike) scales. They have a persistent notochord and 

 a permanently cartilaginous skeleton. There is no air bladder. They are cold 

 blooded (temperature varies with their surroundings). 



Examples: Sharks and skates, rays (Fig. 144). 



Lateral line 



Spinous dorsal fin 



Nasal 

 1 



Soft dorsal fin 

 Caudal fin 



Mandible I 



< I . 



I I Maxillary j 



I Opercle 

 Branchiostegal 



Premaxillary 



Fig. 146. — External features of the perch. (From Hegner: College Zoology. 

 By permission of The Macmillan Company, publishers.) 



Class 3 — Pisces (pis' ez) (L. piscis, fish). — These true fish have gills through- 

 out life which are supported by bony or cartilaginous gill arches. Jaws are present. 

 The two pairs of pectoral and pelvic fins and unpaired median fins are supported 

 by fin rays. The skeleton is principally bony, although cartilage may also be found 

 in certain regions. Scales are present in the skin. The air bladder (swim blad- 

 der) is primarily for hydrostatic purposes, for maintaining a certain level in water 

 without muscular effort. They are cold blooded (their temperature varies with 

 their aquatic surroundings). Their heart is two chambered (1 auricle and 1 ven- 

 tricle). There are about 14,000 species of true fishes. 



Examples: True fishes (Fig. 145), as perch (Figs. 146 and 147), trout, bass, 

 minnow, carp, and goldfish. 



Class 4 — Amphibia (am -fib' i a) (Gr. amphi, both; bios, life). — These 

 aquatic or semiaquatic animals have gills during larval stages and paired lungs 

 in adults (hence amphibious). The skin is slimy, smooth, moist, and usually 



