CHAPTER XIII. 



ARTHROPODA. 



Arthropoda : Animals with jointed appendages, including 

 the following five classes : — 



Crustacea (i). 

 Crabs. 

 Lobsters. 

 Water-fleas. 



\ y 



Y 7 



Mostly aquatic, 

 breathing by 

 gills. 



Arachnoidea (5). Insecta (4). Myriopoda (3). Protracheata (2). 

 Spiders. Centipedes Peripttiits. 



Mites. and Millipedes. 



Scorpions. 



Mostly terrestrial or aerial. 



Mostly breathing by air-tubes or tracheae, hence called Tracheata. 



\Note. — There can be no doubt that the Crustacea and 

 the Tracheata form two divergent series, but within the large 

 and heterogeneous class of Arachnoidea are included the king- 

 crab (Limulus), the " sea-spiders " (Pycnogonida), the extinct 

 Eurypterids and Trilobites, all of aquatic habit.] 



General Characteristics of Arthropods (to which primitive, 

 parasitic, and degenerate forms present exceptions). 



The body is bilaterally symmetrical, and consists of 

 numerous segments variously grouped. Several or all of 

 the segments bear paired jointed appendages variously modi- 

 fied. The cuticle is chitinous. The dorsal brain is con- 

 nected by a ring round the gut with a double chain of 

 ventral ganglia. Above the food-canal lies the heart. The 

 true and primitive coelome is always small in the adult ; the 

 apparent body-cavity is of secondary origin, and has in a 

 great part a blood-carrying or vascular function. The sexes 



