48 DOCTRINE OF ^EVOLUTION 



are not so often met with, and because they are usually 

 very small or microscopic ; but in many respects their 

 importance to the evolutionist surpasses that of the 

 vertebrates. Their structural plans are far more 

 varied, and they range more widely from higher and 

 relatively complicated organisms to the unitary one- 

 celled animals. A knowledge of some of them is es- 

 sential for our present purpose, which is to learn how 

 sure is the basis for the principles of relationship and 

 how complete is the structural evidence of evolution. 



Worms are represented in the minds of most people 

 by the common earthworm or sandworm. The body in 

 either case is made up of a series of segments or joints 

 which agree closely throughout the animal in external 

 appearance and in internal constitution. A section of 

 the digestive tract, a pair of nerve centers, two funnel- 

 like tubes for excretion, and similar blood vessels 

 occur in each portion. 



Precisely similar features are displayed by the Crus- 

 tacea, which seem to be so different. Every one is 

 familiar with the appearance of lobsters and crabs. 

 Even in these animals the body is composed of segments, 

 but these are not like one another, nor are they freely 

 movable throughout the body. Five are fused in all 

 Crustacea to make a head; in lower members of the 

 order the eight succeeding segments are free, but in 

 the lobster they are joined together and united with 

 the head. The hinder part of this animal is a long 

 abdomen whose segments remain more primitive and 

 independent. But in a crab, the whole plan has been 

 modified by the shortening and broadening of the 

 head-thorax, and by the reduction of the abdomen, 



