ARTHROPODA. 189 



The Malacosteaca consist of two groups, the Thora- 

 costraca, which include the most highly organized of 

 the Crustacea • and a lower group, the Arthrocostraca, 

 which are distinguished from them by having- the 

 segments of the thorax (chest region) separate instead 

 of fased, a fact which indicates a more primitive con- 

 dition of the body. Among the Arthrocostraca may 

 be mentioned the Aniphipoda, which include the 

 Sand-fleas, or Sand-hoppers, so common on the sea- 

 shore. They have powerful jaws, and will gnaw 

 through all sorts of things, including sheets left out 

 to dry, when they get the chance. The body of the 

 sand-hopper and its allies is laterally compressed. 

 This is probably an arrangement for their hopping pro- 

 pensities, by which their sharp backs cut the air, as the 

 keel of a ship cuts the water; for they do not jump for- 

 wards like jumping insects, nor backwards like lobsters, 

 but arch their bodies and jump in any direction ; 

 their shape al.so serves another purpose, namely, to en- 

 able them to sidle under stones. An unfailing, however 

 unintentional, trap for them, if their almost incredible 

 hopping powers bring them up into a house near the 

 sea-shore, is a tea-cup and saucer, with a little tea spilt 

 in the saucer. You hear the tea-cup making seem- 

 ingly spontaneous noises, and find that a " hopper " 

 has mistaken the saucer for a wet stone, and insinu- 

 ated itself under the cup, using its sharp back as a, 

 wedge. The task is quite easy for it, for these little 

 Crustacea possess, like beetles, an enormous strength 

 for their size. These interesting creatures do not fail 



