Small Crustaceans. 103 



All these forms make up a sub-class of Crustacea named 

 Podophthalmia on account of their stalked eyes. 



The sand-hoppers (fig. 55), wood- lice, and fresh- 

 water shrimps, make up a second sub-clcss, charac- 

 terised by possessing sessile eyes. Thei-e also have 

 bodies made up of twenty segments, each of \\hich, 

 except those of the head and thorax, has ivs cv,n in- 

 dependent chitinous ring, and the two hinder pairs of 

 foot-jaws are used for locomotion. Some, like sand- 

 hoppers and freshwater shrimps, are laterally com- 

 pressed and have the three hindmost pairs of abdomi- 

 nal feet arranged so that their joints bend forwards while 

 all the other limbs bend with their joints concave back- 

 wards. These are called Amphipoda, to distinguish them 

 from depressed forms like wood-lice, and slaters, whose 

 legs are all directed one way, which are called Isopoda. 



The king crabs of the Mollucca Islands and of 

 North America form the types of a third sub-class. 

 They resemble the lobster in having the head, thorax, 

 and abdomen covered by a great dorsal buckler, but 

 differ in that there are six walking limbs around the 

 mouth, whose bases are spiny, and compressed, 

 acting as jaws. The eyes are not stalked, and there 

 is a long bayonet-shaped tail, behind the post- 

 abdomen, corresponding to the telson of the lobster. 

 The segments of the post-abdomen have a single 

 dorsal shield over them. 



In stagnant pools of fresh water little creatures 

 called water fleas can be seen, by the aid of the micro- 

 scope, actively darting about ; these are representatives 

 of a fourth sub-class. They are all minute at the pre- 

 sent day, but in the past ages of the world, larger allied 



