328 ARTHROPODA. 



Arachnida, Myriapoda, and Insecta, naturally, have not left 

 abundant traces of their existence in past time, a state of 

 things which is assisted by the nature of their integuments, 

 which are rarely as hard and resisting as those of the 

 Crustaceans. 



CLASS I. CRUSTACEA. 



The Crustaceans are Articulate animals in which the breath- 

 ing organs (when distinct) are in the form of gills, and the 

 mode of existence is almost always more or less aquatic. The 

 body is protected by a chitinous or sub-calcareous exoskeleton or 

 " crust" and the member of pairs of articulated limbs is gene- 

 rally from five to seven. Some of the locomotive appendages 

 are often carried upon the segments of the abdomen, and there 

 are two pairs of jointed feelers or " antennae." 



The body of a typical Crustacean, such as a Lobster (fig. 

 194), consists of a definite number of somites placed one 

 behind the other, and divisible into three regions a head, 

 thorax, and abdomen. Most authorities regard the body as 

 being typically composed of twenty-one somites, of which 

 seven go to the head, seven to the thorax, and seven to the 

 abdomen. All these somites, except the last, may.be pro- 

 vided with a pair of appendages each. The last segment of 

 the abdomen, however, never carries any appendages. This 

 segment is known as the "telson" (fig. 194, 1, t), and it is 

 variously regarded as a somite without appendages, or as an 

 unpaired appendage placed in the middle line of the body. 

 If this latter view be adopted, the body of a typical Crusta- 

 cean will consist of only twenty segments, instead of twenty- 

 one. The telson is very greatly developed in some Crusta- 

 ceans, such as the King-crabs, and less so in the extinct 

 Eurypterida. 



Generally speaking, a greater or less number of the somites 

 are amalgamated together, rendering it difficult to recognise 

 their existence unless they bear appendages each pair of 

 appendages indicating a separate somite. Very commonly 

 the segments of the head and thorax are welded together 

 into a single mass, which is termed the " cephalothorax," 



