SECTION XI 

 PHYLUM ARTHROPODA 



In this large and important group of animals we meet with a 

 characteristic feature of the Cha^topoda, viz. metameric segmenta- 

 tion, and also with more or less perfect bilateral symmetry ; the 

 mouth and anus are at opposite ends_jQf-iIie elongated body, and 

 the central nervous system consists of a dorsal brain and a double 

 ventral chain of ganglia. There is, however, an important advance 

 on the segmented Worms in the circumstance that each typical 

 segment bears a pair of appendages, distinguished from the simple, 

 foot-stumps or parapodia of the Polychseta in being divisible into 

 distinct limb-segments or podomeres, separated from one another 

 by movable joints and acted upon by special muscles. Arthropods 

 are also characterised by the almost universal absence of cilia, by 

 their muscles being nearly always of the striped kind, and by the 

 body-cavity largely corresponding not to a true ccelome, but to 

 a hwmoccele, in free communication with the circulatory system and 

 developed from the latter. 



The following are the most important subdivisions of the 

 phylum : — 



Class 1. Crustacea, including Crayfishes, Crabs, Shrimps, 

 Wood-lice, Barnacles, Water-fleas, &c. 



Class 2. Onychophora, including only the curious caterpillar- 

 like Peripatus, and a small number of closely related genera. 



Class 3. Myriapoda, including the Centipedes and Millipedes. 



Class 4. Insecta, including the true or six-legged Insects, such 

 as Cockroaches, Locusts, Flies, Beetles, Butterflies, and Bees. 



Class 5. Arachnida, including Spiders, Scorpions, Mites, &c. 



CLASS I.— CRUSTACEA. 



1. Examples of the Class. 

 a. Apus or Lepidurus. 



Apus and Lepidurus are two closely allied Crustaceans found 

 in the fresh-waters of most parts of the world, but curiously local 



