54 



BRANCH ARTHROPODA. 



acts as a float, though in general, the pedunculated forms 

 are attached to floating objects.* 



The Sessile-eyed Crustaceans differ from those hitherto 

 studied in having the thoracic segments, usually seven 

 in number, each bearing a pair of legs. The eyes are 

 compound,! and placed one on each side of the head, 

 which is not rigidly connected with the thorax. 



FIG. 



ORDER ISOPODA (I sop'6 da). 



The Isopods a' so podz) abound in the sea and 

 in fresh-water; some, like the Sow or Pill Bugs, 

 are terrestrial. The body is flattened, the limbs 

 generally resemble each other (hence the name), 

 and the respiratory (re spir'a to ri) organs are 

 carried beneath the abdomen. The different 

 genera are, for the most part, herbivorous, and 

 hence are of great service in destroying the 

 putrescent matter both in the sea and on the land ; while 

 some are wood-borers, and hence injurious to shipping. 



For eel' li o. 

 Pill-bug. 



ORDER AMPHIPODA (am fip'6 da). 



The Amphipods (am'fi podz) are 

 found in both fresh and salt water, 

 and are particularly abundant on 

 the shore between "tide-marks." 

 They have a compressed body, 

 more or less arched, for spring- 



FIG. 85. 



Or c/ifs' ti a ag' i lis. Beach- flea. 



ing. The gills are borne at the bases of the thoracic feet. 



* Several years since, when the hull of the steamer Great Eastern was cleaned, 

 three hundred tons of barnacles were removed from it. 



t If the eye of some insect be examined with a hand lens, it will be observed 

 to present a multitude of facets, each of which has the value of a single eye. 

 Such eyes are called compound. We have not met with them hitherto, and 

 indeed shall not meet with them except in Arthropoda. 



