SECTION XI 

 PHYLUM ARTHROPODA 



IN this large and important group of animals we meet with 

 a characteristic feature of the Chsetopoda, viz. metameric seg- 

 mentation, as also with more or less perfect bilateral symmetry, 

 mouth and anus at opposite ends of the elongated body, and a 

 nervous system formed 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 parapoclia of the Polychteta in being divisible into 

 distinct limb-segments or podomcrcs, 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, by their 

 sperms being usually non-motile, and by the body-cavity being 

 largely represented by spaces, the blood-sinuses, in free communica- 

 tion with the circulatory system. 



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 a single genus, the 

 curious caterpillar-like Peripatus. 



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. 

 ". 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 in 



