ADVANTAGES OF BEING A VERTEBRATE 235 



thinner chitinous envelope than the more cumbersome "crust" of the 

 crustaceans. Instead of molting at repeated intervals throughout 

 life, they have hit upon the idea of metamorphosis, whereby they do 

 all their molting early during the growing larval stages. Then, as 

 adults of established and unchanging size, they live happily ever 

 after without being troubled by the inconveniences and perils of 

 growth within an unada])tive external encasement. 



Another and paramount objection to a protective exoskcleton is 

 the increasing burden of a heavy armor which soon becomes insup- 

 portable, necessitating a limit to the size of the body encased within 

 it. The largest known representative of the enormous group of the 

 insects is probably smaller even than the smallest adult vertebrate. 



The mxoUuscs have gone at the problem of evolving a skeleton in 

 another way. Although the skeleton is still on the outside, excreted 

 and consequently lifeless, it is never wastefully molted after the 

 crustacean fashion. The parsimonious molluscs keep every particle 

 of their old dead shells and simply add new layers on the inside, as 

 growth demands. The layers, being a little more extensive with each 

 addition, form by their edges the familiar "lines of growth" showing 

 as parallel ridges on the outside of the shell. This particular experi- 

 ment in skeletons, however, has cost the group of molluscs dear, for 

 the heavy shell, together with the accompanying policy of passive 

 defense, has either impeded the power of locomotion with all attendant 

 advantages that would accrue for the evolution of the nervous system, 

 or has brought about its complete abandonment. The clams and 

 their allies, therefore, have stuck conservatively in the mud and 

 lagged behind in the race for life, while other animals without the 

 incubus of a molluscan shell have toiled successfully on to higher 

 levels of attainment in working out their organic salvation. 



The Vertebrate Endoskeleton 



The vertebrates alone exploit a fundamentally different model in 

 skeletal structure. 



An increase in size being necessary for dominance in the struggle for 

 existence, an adequate supporting scaffolding for the body is de- 

 manded, and as a result the skeletal function of protection now be- 

 comes secondary. Levers and muscles to work them to attain 

 locomotion, with ample skeletal surface for their attachment, are also in- 

 dispensable for animals that are to develop a successful nervous sys- 

 tem. The vertebrate skeleton provides for these adaptive advances. 



