CHAPTER XII 



PHYLUM XI. : ARTHROPODA OR JOINTED-LEGGED 

 ANIMALS (LOBSTERS, CRABS, SPIDERS, AND 

 ALL INSECTS) 



Preliminary Amonrst the lower groups of animals, we have 

 Note. noted various means of protection against the two 

 chief classes of danger to which they are exposed — danger 

 from the violence of the dashing waves in the case of those 

 forms living between tide-marks in the sea, and the more 

 widespread danger, occurring in all habitats, of the attack of 

 enemies desirous of devouring anything edible. Some forms 

 seem to flourish owing merely to the simplicity of the 

 demands they make on their environment, and their enor- 

 mous powers of reproduction and regeneration, as in the 

 Protozoa and Hydroid Polyps. Others, such as earthworms, 

 slugs, and snails, escape danger, to some extent, by living 

 in sheltered places and by their cautious habits. The 

 sessile sea-anemones avoid the danger of the violence of the 

 waves by their power of contracting into a solid resistant 

 lump, and escape their enemies through possessing stinging 

 cells, which make them undesirable as food. Bivalve Molluscs 

 and most Echinoderms survive, owing to the secretion by 

 them of a hard protective coat, which either entirely covers 

 their soft body at all times, as in the starfish, or into which 

 the soft body can be withdrawn, as in the mussel and 

 oyster. All such protective shells, however, such as those 

 of the Molluscs, impede motion, and at the same time leave 

 quite unprotected, when in use, the organ of locomotion, the 

 muscular foot. 



The animals of the next group to be considered, the 

 Arthropoda, are segmented like the worms, and are bilaterally 



153 



