SAND AND SHORE HOPPERS 423 



ORDER III.: SAND AND SHORE HOPPERS (AMPH1PODA). 



In streams and rivers are found small crustaceans, ranging from £ to 

 f of an inch in length, which from their appearance have been called 

 Fresh-water Shrimps {Gammarus). They jump by means of the three 

 last large legs of the thorax, which are directed upwards, and the three 

 last abdominal legs, which are bifid. They swim by executing forcible 

 strokes with their movable abdomen. Their progress through the 

 water is assisted by the shape of their body, which is strongly curved 

 and laterally compressed. The three anterior abdominal legs are in 

 constant motion, even while the animal is at rest, being engaged in 

 driving a current of water towards the branchial appendages of the 

 thoracic legs. In the females, a number of plates under the thorax are 

 inclined towards each other, so as to form a cavity for the reception of 

 the eggs. The majority of the Amphipoda live in the sea or on the 

 seashore. 



ORDER IV.: SLATERS AND WOOD LICE (ISOPODA). 



In these crustaceans the body is broad and flattened ; the animals, 

 accordingly, only progress slowly by creeping along the ground, or by 

 rowing themselves lazily through the water. Their branchis are formed 

 by the abdominal legs, which are specially modified for the purpose. 



In the Water Slater (Asellus aquaticus), which is common every- 

 where, these organs may be seen in constant motion. As the terrestrial 

 species (rock slaters and wood lice) also breathe by gills, they live in 

 damp, moist situations. Many, like, e.g., the familiar wood-louse 

 (Oniscus), cannot even live for a night in the air of an ordinary living- 

 room. 



Another species frequently met with is the Armadillo (Armadillidium 

 vulgare). (Compare with pill millipede, p. 405.) Among the Isopoda, 

 the young brood is nurtured and protected in the same way as in the 

 Amphipoda. 



GROUP III.: LOWER CRUSTACEANS (ENTOMOSTRACA). 



If a fine gauze net be drawn through the water of a pond or pool, and 

 then washed into a bowl of water, we shall find that we have captured a 

 quantity of minute creatures. These, in addition to insect larvse, water 

 mites, and infusoria, will be found to consist for the most part of 

 crustaceans, which, however, bear little resemblance to the crayfish. 

 The largest of them do not exceed a fraction of an inch, but many, by 

 reason of the vast quantities in which they occur, are of great importance 



