ANIMALS WITH JOINTED FEET-THE ARTHROPODS 



251 



Fig. 11-51. The millipede (Spirobo/us) possesses two pairs of jointed appendages on most of its segments. These 

 numerous legs move in a rhythmic manner as can be seen from this photograph. Millipedes live in decaying 

 vegetation upon which they feed. 



centipede, which is not only harmless to 

 man but actually beneficial because it feeds 

 exclusively on some of his prime enemies, 

 the insects. 



Millipedes (Fig. 11-51) occupy habitats 

 similar to those of the centipedes and re- 

 semble them in having many legs, except 

 that they possess many more as the name 

 implies. Some have over 100 segments with 

 two pairs of legs on each. When the milli- 

 pede crawls the legs move in a wave-like 

 manner, the wave seemingly moves in a 

 posterior-anterior direction. They are slow, 

 crawling creatures, not at all like the centi- 

 pedes. They feed on plants and decaying 

 organic matter. When in danger some spe- 

 cies roll into a ball, whereas others secrete 

 an offensive fluid which serves them well as 

 a protection against their enemies. 



In summary we have seen that the ar- 

 thropods have acquired a body plan so 



beautifully designed that it has permitted 

 the group to penetrate almost every avail- 

 able type of land and fresh-water environ- 

 ment, and to become the most successful of 

 all animals alive on earth today. The body 

 plan limits the group in only one respect, 

 that of size. All arthropods (with few ex- 

 ceptions) are small animals and to produce 

 successful larger forms an entirely different 

 design had to be evolved. This is beauti- 

 fully accomplished in the chordates, the 

 last group to occupy our attention. How- 

 ever, before going on with this very impor- 

 tant group we must discuss two peculiar 

 groups of animals that appear to represent 

 digressions from the phylogenetic sequence, 

 the mollusks and echinoderms. Their aber- 

 rant body plans, though strange when com- 

 pared to others studied so far, have been 

 sufficiently satisfactory to permit them to 

 spread their kind over much of the earth's 

 surface, both in the water and on land. 



