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Eight Different Classes of Vertebrates Help

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By — McGraw-Hill Professional
Updated on Aug 29, 2011

 Introduction to the Eight Different Classes of Vertebrates

Although all vertebrates share a common core of characteristics, they still show a very wide degree of diversity or differences among them. This diversity is reflected in the existence of eight different classes of vertebrates. These are named the Class Agnatha ( AG -nuh-thuh), Class Placodermi ( PLAK -uh- derm -ee), Class Chondrichthyes (kahn- DRIK -thees), Class Osteichthyes ( ahs -tee- IK -thee-eez), Class Amphibia (am- FIB -ee-ah), Class Reptilia (rep- TIL -ee-ah), Class Aves ( AY -veez), and Class Mammalia (mah- MAY -lee-uh). [ Study suggestion: The eight different vertebrate classes are each labeled with brief representative pictures in Figure 12.4. Before reading any farther in this chapter, see if you can match each of the pictures with the appropriate English translations of their Class names, as follows: the Class of “birds;” the Class of “cartilage-fishes;” the Class of “livers of a double life {on both the land and in the water};” the Class of “jawless” fishes; the Class of animals with “breasts;” the Class of “bony fishes;” the Class of low-“crawling” animals; and the Class of fishes with “flat-surfaced skin.”]

The Chordata: Animals with a “Chord” in Their “Back” Wide Diversity in Backboned Creatures: Eight Different Classes of Vertebrates

Fig. 12.4 The eight major classes of vertebrates.

Class Agnatha and Placodermi

Class Agnatha: Fish-like Vertebrates “without Jaws”

The members of Class Agnatha, fish-like creatures “without” ( a -) “jaws” ( gnath ), were probably among the very first vertebrates to appear on the Earth. Chapter 3 told us that the vertebrates first appeared about 500 million years ago, during the Paleozoic or “Ancient Life” era. It was also during this era that the first amphibians and reptiles evolved.

Class Agnatha, now including about 60 species, mainly consists of the lampreys ( LAM -prees) or “rock-lickers”! Lampreys, of course, do not actually lick rocks! Rather, they are slender, eel-shaped vertebrates with a large, round mouth that attaches to fish and sucks out their body fluids. Lampreys have gill slits, like a fish, but they are not generally considered a type of true fish, since they lack both a hinged jaw, as well as paired fins. (A fish in general is a vertebrate that lives in the water, has a body covered with scales, has a hinged jaw, breathes with use of gills, and has paired fins for swimming.) Due to their primitive characteristics, the modern lampreys probably resemble the first jawless vertebrates in many ways.

Class Placodermi: Extinct Armored Fishes

The Class Placodermi consisted of a group of jawed fishes called placoderms ( PLAK -uh-derms) that are now all extinct! The placoderms were armored fishes with tough, “flat-surfaced” ( placo -) “skin” ( derm ).

Class Chondrichthyes and Osteichthyes

Class Chondrichthyes: Fish With Skeletons Of Cartilage

The Class Chondrichthyes is technically the group of “fishes” ( ichthy ) with skeletons made of “cartilage” ( chondr ). This group chiefly includes the sharks and sea rays. Even though their skeleton is made of relatively soft cartilage, the sharks have very hard teeth that are bony. These cartilaginous ( kar -tih- LAJ -ih-nus) fishes all have hinged jaws and paired fins.

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