fHE COMING OF A BACKBONE 27 



sufficient nourishment to complete the growth and develop- 

 ment from the larval to the adult stage. 



If the metamorphosis or series of changes from larva to 

 adult is comparatively easy it requires no long resting stage; 

 even if great and difficult, it may be distributed along the 

 successive moults, or all be condensed in the cocoon or rest- 

 ing stage of the so-called complete metamorphosis. This ex- 

 plains the voracity and destructiveness of insect larvae. It 

 does not surprise us to find that the most destructive insect 

 pests are usually the smallest, like the plant lice, for example. 

 Where limited locomotion or sessile life is added to small 

 size, as in the scale-bugs, the insect becomes a veritable 

 scourge. 



Insects disappoint us. Their good qualities and powers 

 are many and marked; their limitations are equally clear and 

 in time seem to have put an end to progress. The external 

 skeleton gives good returns for a long time, but leads to 

 standstill in the end. Life must and did produce something 

 capable of surer and less limited progress. 



The Vertebrate. The internal locomotive skeleton. 



The ancestors of vertebrates or backboned animals, the prim- 

 itive chordata, neglected all the advantages of protective armor, 

 and placed the skeleton inside the body as near its axis as the 

 large vegetative organs permitted. The primitive skeleton 

 was a rod of spongy tissue which, for lack of a better name, 

 we call cartilage. It is sheathed by a layer of fibrous con- 

 nectile tissue, which in time gave place to true cartilage. 

 This sheath sends projections upward to arch over the dorsal 

 nerve-cord and downward around the great blood vessels. 

 Sheath and arches furnish attachment for two heavy bands or 

 masses of muscles running lengthwise on each side of the body. 

 These may at first have produced a writhing motion, but later 

 they pulled the tail-fin right and left alternately and sculled 

 the body through the water. 



The head is large with eyes, ears and nasal capsules. The 



