OF CENTRAL CANADA PART IV. 263 



or short-tailed decapods, in which the abdomen is rudimentary and 

 bent under the cephalo- thorax. They are represented by the various 

 crabs, properly so-called. Fossil forms have been cited from Palaeo- 

 zoic and Jurassic rocks, but the earliest undoubted examples are 

 Cretaceous. 



II. 



ARACHNIDA. 



This Class is represented typically by Spiders, Harvest-Spiders. 

 Chelifers and Scorpions, in which the respiration is aerial (tracheal 

 or pulmo-branchial), the body and thorax united, and the legs in four 

 pairs. In certain lower forms there are no visible organs of respira- 

 tion, and the legs are variable in number. 



In Spiders, the abdomen is short and unsegmented. Examples 

 date from the Carboniferous period. 



Scorpions have a long abdomen divided into segments and scarcely 

 distinguishable from the thorax. They possess also a pair of large 

 antennae or palpi furnished at their extremities with nipper-claws, 

 as in the extinct crustacean genus Pterygotus. From this and other 

 characters, the latter is referred to the Scorpions by some palaeonto- 

 logists. The earliest known Scorpions appear in Carboniferous strata, 

 in which, also, examples of chelifers are said to have been discovered ; 

 but no fossil arachnids belong to our rocks. 



III. 



MYRIAPODA. 



The myriapods are represented essentially by centipedes and milli- 

 pedes, in which the form is elongated and worm-like, the abdomen 

 being divided into numerous segments each of which bears a pair or 

 a couple of pairs of short legs. The head is distinct from the thorax, 

 and the respiration is trachseai. Fossil examples first appear in 

 Carboniferous strata, but are of rare occurrence. None occur in our 

 rocks, but a species of Xylobius (allied to lulus] was recognized 

 by Sir J. W. Dawson, many years ago, in the hollow stem of a sigil- 

 laria from the middle or productive coal formation of Nova Scotia. 



