GENERAL LAWS OF ANIMAL STRUCTURE. 



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I. GENERAL PLAN OF STRUCTURE GENERAL 



HOMOLOGY. 



i. All vertebrates, and no other animals, have an 

 interior skeleton, with the muscles acting on it from the 

 outside to produce motion and locomotion. 



2. In all vertebrates, and in no other animals, the 

 axis of this skeleton is a jointed backbone or vertebral 

 column. It is this that gives name to this department. 

 They are vertebrates or backboned animals. 



3. In all vertebrates, and in no other, this axis in- 

 closes two cavities : one, the neural cavity, n, is above, to 

 protect the nervous centers; 

 and the other, the visceral cav- 

 ity, v, below to inclose the vis- 

 ceral centers (Fig. 159). 



4. In all vertebrates, and 

 in no other, the head may be 

 regarded as a coalescence of 

 several vertebral joints.* 



5. In nearly all vertebrates, 

 and in no other, there are 

 two, and only two, pairs of 

 limbs. The exceptions to this 

 law fall under two categories 

 — viz., those like the lowest 

 fishes, which represent the 

 condition before limbs were 

 added to the skeleton, and 

 those like snakes and like some lizards and amphibians, 

 in which the limbs have been lost. 



This general common plan of skeletal structure 

 strongly suggests a common origin. 



Fig. 159. — Cross section through 

 a fish : v s, visceral system ; 

 vert, vertebra ; d, blood sys- 

 tem ; ns, nervous system. 



* Some do not hold this view of the origin of the head. 



