THE ANIMAL CLIMAX-THE CHORDATES 



303 



ily vascularized for its function in breath- 



ing 



The dermal scales of the fish are notice- 

 ably absent among the modern amphibians, 

 although fossil remains indicate that their 

 ancestors were well covered with scales. 

 Scales offer excellent protection from attack 

 and it seems sti-ange that the amphibians 

 have given up this apparently valuable aid 

 in self-preservation. It must be remem- 

 bered, however, that in present-day species 

 the skin is an "accessory" lung and very im- 

 portant in respiration. Only because of this 

 condition is it possible for the amphibian to 

 spend its quiet periods in an environment 

 where breathing with lungs is impossible. 



The locomotor organs. When the loose 

 skin of the frog is removed, a set of muscles 

 is exposed that far surpasses anything pos- 

 sessed by fish. The most conspicuous differ- 

 ence noted between the froo; and fish is the 

 remarkable development of the muscles 

 that operate the appendages, which are re- 

 sponsible for the agile jumping movement 

 on land. Although many of the muscles ap- 

 proximate the position and seem to func- 

 tion much the same as similar muscles in 

 man, most comparative anatomists agree 

 tliat only a very few of them are identical. 

 Apparently they are derived from similar 

 muscle masses of ancestral forms but prob- 

 ably followed different lines of evolution. 

 The muscles of the frog are named in Fig. 

 13-28 and should be studied in terms of 

 their function in locomotion and not from a 

 comparative point of view. 



Muscles are contractile tissues which 

 function much like rubber bands. They are 

 always under slight tension in life, even 

 when relaxed. They differ from the rubber 

 band analogy in that they have the power 

 to contract violently when stimulated. The 

 contractile portion is the fleshy or "belly" 

 part of the muscle which is attached to the 

 bones by tendons. The latter are fibrous, 

 tough, and non-contractile. A muscle is 

 identified by noting its origin, which is the 

 end that moves less when contraction oc- 



curs, and its insertion, the end which moves 

 more. Muscles are attached to the bones in 

 many positions, and they vary in size from 

 the tiny muscles that close the eyelids to the 

 large muscle that extends the leg. It is the 

 great variety both in the muscles themselves 

 and their points of attachment to the bones 

 that make possible all of the many move- 

 ments made by the frog. When such a sys- 

 tem is carried to higher animals, man, for 

 example, it is evident that there must be a 

 great many muscles to carr\' out the many 

 and complicated movements of which such 

 a form is capable. 



The supporting structure. Although the 

 internal skeleton of the frog is made of bone 

 and in many respects resembles that of 

 man, in other respects it must be considered 

 as the skeleton of a "specialized" verte- 

 brate ratlier than a "generalized" form be- 

 cause it differs so markedly from primitive 

 vertebrates (Fig. 13-29). Perhaps its most 

 conspicuous loss it that of ribs and a tail 

 which the majority of primitive vertebrates 

 possess, but which the fiog, for some rea- 

 son, has lost. Furthermore, its body is much 

 foreshortened with the loss of many verte- 

 brae. Most vertebrates have from 20 to 30 

 vertebrae, whereas the frog has only nine. 



The appendages are attached to the ver- 

 tebral column by means of girdles, the pec- 

 toral in front, the pelvic behind. These are 

 c^uite generalized and hence much like 

 those in most other vertebrates. The pecto- 

 ral girdle consists of three principal pairs of 

 bones attached to a series of midventral 

 bones called the sternum. The scapulae are 

 located on the dorsal side of the trunk (the 

 flat extension is called the suprascapula) 

 and this structure is similar to the human 

 shoulder blade. It joins ventrally with the 

 clavicle and coracoid which in turn fuse 

 to the sternum. The clavicle (collar bone) 

 is well developed in man but the coracoid is 

 only a small "bump," fused to the scapula. 

 The pelvic girdle is composed of three pairs 

 of bones, which in the adult are fused into 

 a single structure. The long, flat, anteriorly 



