102 DIVERSIONS OF A NATURALIST 



harder pieces, thus rendered capable of being bent or 

 " flexed " on one another. Thus the body is jointed into 

 a series of rings, and the legs are also divided each into 

 several joints (as many as seven), which gives them 

 flexibility and so usefulness of various kinds. The 

 various joints are " worked " by powerful muscles, which 

 are fixed internally to the cuticle and pass from one hard 

 ring or segment, whether of body or of leg, to a neighbour- 

 ing ring. 



Every one knows the structure of a lobster's tail and 

 of its legs, which can be readily examined in illustration 

 of my statement, and the same structure can be seen in 

 the leg of a beetle or a fly. Naturalists term all this 

 series of creatures with hard -jointed cuticle, to which the 

 muscles are attached, including the crab class, the insects, 

 centipedes, spiders, and scorpions, "jointed-leg owners," 

 or Arthropods. It is easy to appreciate this characteristic 

 difference which separates the Arthropods from other 

 animals. The sea-worms differ from them, in that they 

 have soft cuticle, but stiffen and render their paddle-like 

 legs firm by squeezing the liquid of the body into them 

 in the same sort of way as the sea-anemones distend 

 their tentacles with liquid, though in that case the liquid 

 is sea-water taken in by the mouth. The Molluscs also 

 distend their muscular lobe, or " foot " as it is called, by 

 pressing the blood from the rest of the body into it, and 

 so making it swell and become stiff, so that the muscles 

 can work it ; when not distended in that way it is flaccid. 

 The Vertebrates (bony animals) and the star-fishes have 

 again another and peculiar mechanism. Their muscles 

 are attached to hard internal pieces, sometimes cartila- 

 ginous but often calcareous or bony, which are spoken of 

 as " the internal skeleton." There are thus three distinct 

 kinds of mechanism in animals for giving the necessary 



