PHYLUM ARTHROPODA 125 



c) Shrimps, marine animals similar to crayfishes, but with a more slender, 

 delicate build. 



d) Crabs, marine animals like the lobster, but with a much broadened 

 cephalothorax, and reduced abdomen folded under the cephalothorax. 



e) Amphipods, small fresh- water animals, something like miniature cray- 

 fishes, but strongly compressed from side to side. 



/) Sow bugs or pill bugs, small forms, greatly compressed dorsoventrally, 

 common in damp places, greenhouses, etc., also in water; curling into balls 

 when disturbed. 



2. Arachnids, forms without antennae, and four pairs of walking legs. 



a) Spiders, require no description. 



b) Scorpions, with segmented abdomens narrowing to a long tail, bearing a 

 terminal sting. 



c) Daddy longlegs, or harvestmen, with small bodies, indistinctly segmented, 

 and very long slender legs. 



d) Mites, small, flattened forms, having lost all traces of segmentation, 

 generally external parasites. 



3. The king crab, or horseshoe crab, survivor of an ancient, mostly extinct, 

 group, with a large rounded cephalothorax; abdomen terminated by a long 

 sharp spine. 



4. Insects, with one pair of antennae, three pairs of walking legs, breathing 

 by means of tracheae. 



5. Millipedes and centipedes, elongated forms consisting of many segments. 

 Each segment has one pair of legs in the centipedes, and two pairs in the milli- 

 pedes. 



