CHAPTER III 



A STUDY OF LIVING SUBSTANCE 



1. THE GENERAL STRUCTURE OF ANIMAL BODIES 



Vertebrates and Invertebrates. All animals may be 

 divided into two great classes, known respectively as the 

 ver'te-brates and the in-ver'te-brates. To the first group belong 

 the animals that have a backbone. We are all familiar 

 with common vertebrates like fishes, frogs, snakes, birds, 

 and cats. Insects, worms, and clams, on the other hand, 

 have no backbone; hence they are called invertebrates 

 (i.e. animals without vertebrse). 



Regions of the Body. In man and in most other verte- 

 brates we can distinguish the head and neck region, the 

 trunk, and the four ap-pend'ag-es or limbs which are at- 

 tached to the trunk, namely, two arms and two legs, or, as 

 they are more often called in descriptions of the lower ani- 

 mals, the four legs. Since frequent reference will be made 

 in the following pages to different vertebrates and inverte- 

 brates, we must become familiar at the outset with certain 

 terms that will locate definitely corresponding regions in all 

 animals. 



Man walks on two appendages (the legs) ; the long axis of 

 his trunk is vertical ; and above his body is his head. But 

 dogs and other four-footed animals have a horizontal trunk 

 with the head attached in front. Hence the same adjective 

 cannot be used to describe the position of the head of man 

 and of quadrupeds. Biologists have, therefore, adopted the 

 term an-te'ri-or (Latin an'te = before) which can be applied 



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