460 ARTICULATES I CRUSTACEANS. 



SCOLOPENDRIM:, Leach, OR CENTIPEDE FAMILY. 

 This Family comprises chilopods represented by the 

 Genus Scolopendra, which has twenty rings besides those 

 that form the head. Our species are only two or three 

 inches long ; but in the warm and tropical regions the) 

 are much larger, even a foot in length in some cases. 

 The bite of the tropical species is very poisonous. 



GEOPHILID^:, Leach. This Family comprises chilo- 

 pods which are greatly elongated and slender, and with 

 one or two hundred rings in some cases. 



Fossil Insects are found in the rocks as low as the 

 Devonian inclusive. 



SECTION II. 



THE CLASS OF CRUSTACEA, OR CRUSTACEANS. 



THE Class of Crustacea includes all articulated animals 

 which have the head and thorax essentially in one piece, 

 called cephalo-thorax, and which respire by means of 

 gills, being thus aquatic in their mode of respiration, and 

 which have a straight alimentary canal, and shed, at more 

 or less regular intervals, their usually solid calcareous 

 covering or external skeleton. The body of a Crustacean 

 consists normally of twenty-one segments, fourteen be- 

 longing to the cephalo-thorax, and seven to the abdomen ; 

 but in the adult these are not generally apparent. These 

 animals are carnivorous, mainly aquatic, and mostly ma- 

 rine ; but some live on the land, others in fresh water, and 

 all can remain out of water for a considerable time with- 

 out perishing. The locomotive organs of Crustaceans are 

 very numerous ; for in many cases every segment has its 

 pair of appendages ; but notwithstanding all these append- 

 ages have the same fundamental structure, they are spe- 

 cialized so as to perform very various functions, as those 



