ANNULOSA: MYRIAPODA. 327 



metamorphosis ; but they cast their skins or moult repeatedly, 

 before they attain the size of the adult. Most Spiders deposit 

 their eggs in silken nests or cocoons, often beautifully con- 

 structed, and sometimes carried about by the females. The 

 males are generally smaller than the females, and of rarer 

 occurrence. 



DISTRIBUTION OF ARACHNIDA IN TIME. The Arachnida 

 are only very rarely found in a fossil condition. As far as is 

 yet known, both the Scorpions and the true Spiders appear 

 to have their commencement in the Carboniferous epoch, the 

 former being represented by the celebrated Cyclophthalmus 

 senior from the coal-measures of Bohemia, and by the Eoscorpius 

 carbonarius of the Carboniferous strata of Illinois. Other 

 Carboniferous Arachnida have been referred to the genera 

 Eophrynus, Architarbus, and Mazonia. Spiders are also known 

 to occur in the Jurassic rocks (Solenhofen Slates) and in the 

 Tertiary period. The Mites, Harvest-spiders, and Book-scor- 

 pions have been detected in amber. 



CHAPTER XXXVII. 



MYRIAPODA, 



CLASS III. MYRIAPODA. The Myriapoda are defined as ar- 

 ticulate animals in whith 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 antenna, and the number of the legs is always more 

 than eight pairs. Respiration is by trachece. 



In this class comprising the Centipedes (figs. 169, 170) and 

 the Millepedes 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 segments 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 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, 



