XI PHYLUM ARTHROPODA 653 



features of their own ; but different families of the order approxi- 

 mate to a certain extent towards the groups of living Insects. 

 Amongst them, for example, are forms representing the Cock- 

 roaches and the Phasmidse among the Orthoptera ; others repre- 

 senting the modern Day-flies among the Neuroptera ; others the 

 Coleoptera. 



Of the existing orders, the Neuroptera, Orthoptera, and Coleop- 

 tera are first found in the Trias ; the Hemiptera, Diptera, Hymen- 

 optera, and Lepidoptera in the Jurassic. 



CLASS V.-ARACHNIDA. 



The class Arachnida, comprising the Scorpions and Spiders, the 

 Mites and Ticks, the King-crabs, and a number of other families, 

 is a much less homogeneous group than the Insecta, approaching 

 the Crustacea in the variety which it presents in the arrangement 

 of the segments and their appendages. In most members of the 

 class, however, there is an anterior region of the body — the cephalo- 

 thorax — representing both head and thorax, and a posterior part, 

 or abdomen, which is typically composed of a number of distinct 

 segments ; in some cases cephalothorax and abdomen are amalga- 

 mated. There are no antennae in the adult Arachnid, though 

 rudiments of them have been found in the larvae of some species. 

 The first pair of appendages of the cephalothorax (probably repre- 

 senting the antennae of the Crayfish) are the chelicerce ; the second 

 are the pedipalpi, the representative of the Crayfish's and Cock- 

 roach's mandibles. Behind these are four pairs of legs. The 

 organs of respiration are sometimes tracheae, similar to those of 

 the Insects, sometimes book-lungs, or sacs containing numerous 

 book-leaf-like plates : sometimes leaf-like external appendages 

 or gills. 



1. Example of the Class. — The Scorpion {Uuscorpio or 



Buthus). 



Scorpions are inhabitants of warm countries — the largest kinds 

 being found in tropical Africa and America. They are nocturnal 

 animals, remaining in holes and crevices during the day, and 

 issuing forth at night to hunt for their prey, which consists of 

 Spiders and Insects. These they seize with their pincer-claws 

 and sting to death with their caudal spine, afterwards sucking 

 their juices. 



There are a number of different species of Scorpions, divided 

 into several genera, which differ from one another in compara- 

 tively unimportant points, so that the following general descrip- 

 tion will apply almost equally well to any of them, 



