408 THE MOUNTAIN. 



he, as may be seen at almost any time, groping around on 

 the bottom of runs, or sliding, head or tail first, as best 

 suits him, along the edges of stones and roots, or in the 

 mud. He is rather pugnacious, and very quickly seizes on 

 anything that falls in the water. It has a wide geographic 

 range, and is exclusively fluviatile. Length, 3J inches. 



ORDER II. STOMAPODA. 



This is divided into two families, Unipeltata, Bipeltata, 

 and into genera and species, all of which are marine. 



MALACOSTRACA, group 6, eyes sessile and immovable. This 

 division of crustaceans is small. They inhabit the sea-coast, 

 also fresh water, and some are terrestrial and parasitical. 



ORDER III. AMPHIPODA. 



Most of this order inhabit salt water ; some, however, 

 are found in springs and rivulets. Some of this group, be- 

 longing to the sea, are the most ferocious and ravenous of 

 the crustaceans. 



GAMMARUS minus. Fresh-water shrimp. This is a com- 

 mon animal in fresh-water streams under stones and roots. 

 " It is quick and lively in its motions, and altogether an 

 interesting creature. " 



GAMMARUS fasciatus is quoted as a Pennsylvania species. 



ORDER IY. LOSMIPODA, (Throat-footed.) 

 This order is exclusively marine. 



ORDER Y. ISOPODA, (Equal-footed.) 

 Most of this order are aquatic, some terrestrial, and some 

 are parasitical. Among the terrestrial are the species of the 

 genus Oniscus, or wood-lice. 



ASELLTJS communis. This is a little fresh-water crus- 

 tacean, common in runs and found under stones. 



ONISCUS asellus. The sow-bug, or, as it is commonly 



