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CHAPTER XVII. 



ARTHROPODA. 



CRUSTACEA. 



DIVISION ARTHROPODA, or ARTICULATA. The members of 

 this division of the sub-kingdom Annulosa are .distinguished 

 from the preceding division of the Anarthropoda by the pos- 

 session of jointed appendages, articulated to the body. The 

 body is composed of a series of segments or " somites," arranged 

 along a longitudinal axis ; each segment occasionally, and some 

 always, being provided, at some period of life or other, with 

 articidated appendages. Both the segmented body and the 

 articulated limbs are more or less completely protected by an 

 external skeleton formed by the deposition of horny (chitinous) 

 matter, sometimes along with variable amounts of carbonate 

 and phosphate of lime, in the integument. 



The division Arthropoda includes four great classes of 

 animals which are very generally spoken of as the " Articu- 

 late Animals." These classes are the Crustacea (Lobsters, 

 Crabs, &c.), the Arachnida (Spiders, Scorpions, &c.), the 

 Myriapoda (Centipedes and Millipedes), and the Insecta (In- 

 sects). All these classes came into existence in the Palaeo- 

 zoic period, the division being represented by Crustaceans as 

 early as the Upper Cambrian at any rate, and doubtfully in 

 the Lower Cambrian. Owing to the fact that the Crusta- 

 ceans alone lead an habitually aquatic life, the remains of 

 this class, as might be expected, preponderate largely over 

 those of the other three. The air-breathing classes of the 



