ANNULOSA: MYRIAPODA. 



245 



CHAPTER XXXVIII. 



MYRIAPODA. 



Class III. Myriapoda. — The Myriapoda are defined as ar- 

 ticulate animals in which the head is distinct, and the remainder 

 of the body is divided into nearly similar segments, the thorax 

 exhibiting no clear line of demarcation from the abdomen. There 

 is one pair of antennce, and the number of the legs is always more 

 than- eight pairs. Respiration is by trachece. 



In this class— comprising the Centipedes (fig. 8i) and the 

 Millipedes — the integument is chitinous, the body is divided 

 into a number of somites provided with articulated 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 invariably distinct, and there is no 

 marked line of demarcation between the seg- 

 ments of the thorax and those of the abdomen. 

 The body, except in Pauropus, always consists 

 of more than twenty somites, and those which 

 correspond to the abdomen in the Arachnida 

 and Imecta are always provided with locomo- 

 tive limbs. " The head consists of at least 

 five, and probably of six, coalescent and modi- 

 fied somites ; and some of the anterior seg- 

 ments of the body are, in many genera, coal- 

 escent, and have their appendages specially 

 modified to subserve prehension." — (Huxley.) 

 Pauropus has only nine pairs of legs ; but with 

 this exception, eleven pairs of legs is the small- 

 est number known in the order. 



The respiratory organs, with one exception, 

 agree with those of the Insecta and of many of 

 the Arachnida in being " tracheae" — that is to 

 say, tubes, which open upon the surface of the 

 body by minute apertures, or " stignaata," and 

 the walls of which are strengthened by a spirally- 

 coiled filament of chitine. The trachese may 

 or may not anastomose with one another as 

 they do in Insects. 



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

 abdominal segment, are usually undistinguishable from one 

 another, and each bears a single pair of limbs. In some cases, 



Fig. 81. — Centipede 



