CHAPTER XVIII 



The Arthropods 



( Phy hi m A / /// / opoda ) 



(Upper left) Orb-weaving spider and horseshoe crab; (upper right) 

 velvet worm and lobster; (lower left) centipede; (lower right) bear 

 animalcule and water flea 



o, 



F all the invertebrates, it is only members of 

 the phylum Arthropoda that have mastered flight. In- 

 deed, no other phylum of animals is so widely dis- 

 tributed, from pole to pole, from the greatest abysses 

 to the highest peaks, from glacier to boiling spring, 

 as well as from the driest desert and saturated salt 

 lake to the felt of moss kept perpetually wet by the 

 misty spray from a waterfall. 



It is tempting to refer to the arthropods simply as 

 "the insects and their kin," for the insects, which be- 

 long to this phylum, constitute three-quarters of the 

 known kinds of animals. With other arthropods, in- 

 sects share the possession of an external skeleton 

 composed of hard cuticle, and jointed legs — the fea- 

 ture from which the entire phylum derives its name. 

 Typically, the hard cuticle is molted at intervals, al- 

 lowing the animal to grow by a succession of steps 

 during the few hours while the body wall is flexible. 

 Typically, too, each segment of the body bears a pair 

 of jointed limbs. Commonly each leg ends in a pair 

 of claws. 



By interpreting these features liberally and watch- 

 ing for others that are found in arthropods about 

 which no argument can be raised, it is possible to 

 gather into this huge phylum two minor groups — • 

 the tardigrades and the onychophorans — which show 

 affinities to other phyla as well. The remaining ar- 



thropods can be subdivided easily into those with 

 true jaws and those without. 



The "mandibulate" arthropods have as append- 

 ages a pair of mandibles that do not end in claws. In- 

 stead, they work from side to side as chewing or 

 crushing organs, or are modified in ways making 

 them useful in piercing or sucking. The mandibulates 

 also have antennae, either two pairs as in crusta- 

 ceans or one pair as in centipedes, millipedes, and 

 insects. Moreover, their body appendages tend at 

 the base to show a forked character. 



Arthropods without true jaws are spoken of as 

 "chelicerate" because they all have associated with 

 the mouth a pair of pincer-tipped appendages, the 

 chelicerae. Chelicerate arthropods all lack antennae. 

 Their body appendages are never forked at the base, 

 and the first pair behind the chelicerae are usually 

 modified into fingering organs, the pedipalps. Chelic- 

 erate arthropods include the marine horseshoe 

 "crabs," the arachnids (the terrestrial spiders, scor- 

 pions, ticks, and mites), and the sea spiders. 



The Bear Animalcules 



( Class Tardigrada ) 



An almost infallible method for collecting live 

 bear animalcules can be applied after a day or two 



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