234 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



getting about on land, in water, and in air, far surpassing those 

 employed by lower animals. 



Skeletons 



Let us consider briefly just one vertebrate feature, namely, the 

 living inside skeleton, which gives the name "vertebrate" to the group. 

 It is the culmination of an endless array of experiments and adapta- 

 tions that have been going on since the beginnings of life on this 

 planet, and there is every indication that the end is not yet. The 

 skeleton of man, for example, is by no means the final mechanism of 

 its kind. There are to be expected in the future other models nearer 

 to perfection, though based upon all that has gone before. 



Invertebrate Attempts 



The microscopic protozoa made brave experiments with the idea 

 of a skeleton, in their case an armor mostly for protective pur- 

 poses and consequently found located on the outside of the animal 

 itself. In fact, protection seems to be the primary service of skeletal 

 structures in general, although secondarily supplanted largely by the 

 function of support and muscle attachment. It still plays an important 

 role even in the vertebrates, since the brain and cord, being ex- 

 tremely liable to injury, are enclosed within a protective skull and 

 enveloping vertebral arches, while the viscera are in part stowed away 

 within a bony thoracic basket. 



In the great group of arthropods, that includes both crustaceans 

 and insects, the skeleton is plainly a protective external covering 

 which, being a lifeless excretion of the skin, does not change in size 

 after it has been laid down. As the anima'l grows, the dead inelastic 

 skeletal armor thus formed fits more and more tightly over the 

 enlarging body until finally it has to crack open in order that the 

 animal may emerge and become refitted, after an interval of rapid 

 bodily expansion, with a new and larger skeletal garment. This 

 complicated process is called molting. To elaborate and then periodi- 

 cally to reject all this material is not only a physiologically expensive 

 performance but it is also a hazardous one, since while shifting into 

 a more commodious suit of armor, the animal may lose a leg or two, 

 and is always exposed for some time to enemies while in its defenseless 

 shell-less condition. 



Insects, caught in the same evolutionary blind alley with their 

 crustacean cousins, have taken an upward step by secreting a much 



