206 



MANUAL OF ZOOLOGY. 



former being represented by the celebrated Gyclophthalmus se- 

 nior from the Coal-measures of Bohemia. Spiders are, also, 

 known to occur in the Jurassic Bocks (Solenhofen Slates) 

 and in the Tertiary period. 



CHAPTER XXXVIII. 

 MYRIAPODA. 



CLASS III. MYRIAPODA. The Myriapoda are denned as articu- 

 late animals in which the head is distinct, and the remainder of 

 the body is divided into nearly similar seg- 

 ments, the thorax exhibiting no clear line of 

 demarcation from the abdomen. There is one 

 pair of antennce, and the number of the legs 

 is indefinite. Respiration is by tracheae. 



In this class comprising the Centipedes 

 (fig. 60) and the Millipedes the integu- 

 ment is chitinous, the body is divided into 

 a number of somites provided with articu- 

 lated appendages, and the nervous and 

 circulatory organs are constructed upon 

 a plan similar to what we have seen in 

 Crustacea and Arachnida. The head is in- 

 variably distinct, and there is no marked 

 line of demarcation between the segments 

 of the thorax and those of the abdomen. 

 The body always consists of more than 

 twenty somites, and those which corre- 

 Fig. 60. Centipede (Sco~ spond to the abdomen in the Arachnida and 



lopendra) reduced. T r , . , -. ., , , , . 



Insecta are always provided with locomotive 

 limbs. * The head consists of at least five, and probably of 

 six, coalescent and modified somites, and some of the anterior 

 segments of the body are, in many genera, coalescent, and 

 have their appendages specially modified to subserve prehen- 

 sion.' (Huxley.) 



The respiratory organs agree with those of the Insecta and 

 of many of the Arachnida in being * tracheae,' that is to say, 

 ramified tubes, which open upon the surface of the body by 

 minute apertures, or * stigmata,' and the walls of which are 

 strengthened by a spirally coiled filament of chitine. 



The somites, with the exception of the head and the last 

 abdominal segment, are usually undistinguishable from one 



