CH. XIl] MAIN DIVISIONS 1*75 



The exoskeleton not only furnishes an external armour but 

 penetrates between the limbs of inturned folds of ectoderm termed 

 apodemes into the interior of the body and furnishes support for 

 the muscles. 



The Arthropod* may be divided into the following classes : 



I. The CRUSTACEA, possessing two pairs of feelers (antennules 



and antennae) and three pairs of jaws which include 

 all the Crabs, Lobsters, Crayfish, Barnacles, Wood- 

 lice, etc., besides countless small forms such as the Water-flea, 

 Cyclops, and many others which inhabit both salt and fresh 

 water. 



II. The ANTENNATA, which include all Arthropoda possessing 

 one pair of feelers antennae and breathing by means of air tubes 

 or tracheae. This group is divided into three sub-classes, viz. : 



A. The Prototracheata, a group containing the genus 

 Peripatus, an animal riot found in Europe or in North America, 

 but which must be mentioned because it seems to be a survival from 

 an earlier age and because its structure has given us a clue to much 

 that was obscure in the anatomy of Arthropods: it is in fact in 

 many respects intermediate between the Annelids and the air- 

 breathing Arthropoda. 



B. The Myriapoda or Centipedes, the commonest British 

 examples of which are the chestnut-coloured centipede Lithobius 

 forficatus and the black "wire-worm 1 " lulus terrestris. 



C. The Insect a, the largest group in the Animal Kingdom. 

 It contains about 250,000 named species, and includes all those 

 creatures such as Beetles, Flies, Dragon-flies, May-flies, Moths, Bees, 

 Ants, Wasps, etc., which we are accustomed to call insects. 



III. The ARACHNIDA, devoid of feelers but having a small 

 pair of claws (chelicerae) as first appendages, and having as jaws mere 

 processes of their walking legs. This class includes the Spiders, 

 Harvestmen, Mites and certain larger forms such as the Scorpion, 

 and Limulus, the King-crab. 



IV. The PANTOPODA, with chelicerae but devoid of jaws 

 altogether,-a group of small animals incorrectly termed "sea-spiders." 

 They are marine and may be found under stones between tide marks 

 on our coast. 



V. The TARDIGRADA or "Bear Animalcules." These are 

 minute microscopic Arthropoda found amongst moss and decaying 



1 This is not to be confused with the larva of a beetle, Elater lineatus, which 

 is also called a " wire- worm" by the British agriculturist. 



