MORPHOLOGY OF ANIMALS 



and in addition dermal bones are formed which partially 

 overlie this cartilaginous basis. The appendicular skeleton 

 consists of two limb girdles partially surrounding the trunk 

 and axial skeleton, namely the pectoral and the pelvic 

 girdles^ and of the skeleton of the limbs themselves (Fig. 

 8). In the fishes these girdles and limbs are peculiar and 

 it is difficult to homologize their skeletal parts with those 

 of higher forms; in all vertebrates above the fishes, how- 

 ever, the relations of these parts are similar and their 

 homologies are not difficult to determine. 

 The corresponding parts and bones of the 



3. Motor System 



All animals at some time in their lives have the power of 

 locomotion, though in some cases this is lost before adult 

 life is reached and the animal becomes fixed like a plant 

 (hydroids, sponges, crinoids, molluscoids, and many para- 

 sites). However, in all these cases certain parts of the 

 body preserve the power of movement, though the animal 

 as a whole is incapable of locomotion. Animal movement 

 is of three fundamental types: amoeboid, ciliary, and 

 muscular. 



(^) Amoeboid movement is limited to individual cells 

 and is manifested especially by free cells. It consists of a 



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