CHAPTER XIII. 

 AETHEOPODA. 



DIVISION II. AETHEOPODA or AETICULATA. The members 

 of the sub-kingdom Annulosa comprised under this head are 

 generally known as Articulate animals, or as Arthropoda (Gr. 

 arthros, a joint ; and podes, feet). They are all distinguished 

 by the possession of jointed appendages articulated to the body. 

 The body is composed of a series of distinct rings or segments 

 (technically called " somites ") arranged longitudinally one be- 

 hind the other. The skin is more or less completely hardened 

 by a horny deposit of " chitine," with or without lime, so as to 

 form a resisting shell, to the inner surface of which the muscles 

 are attached. There is consequently no necessity for any in- 

 ternal skeleton. The nervous S3 r stem in the young of all 

 Articulate animals has its typical form of a chain of ganglia 

 placed along the ventral surface of the body, and traversed in 

 front by the gullet. In the adult, however, this typical state 

 of the nervous system is often lost or modified. The blood- 

 circulatory system may be absent ; but, when it is present, it is 

 placed dorsally (Fig. 44), and consists of a true blood-system 

 containing corpusculated blood, and furnished with a contractile 

 cavity or heart. Respiration is sometimes effected simply by 

 the general surface of the body, but there are generally special 

 organs adapted for breathing air, either directly or through the 

 medium of water. Jointed appendages are always present, and 

 may be developed from any segment of the body. 



The Arthropoda are divided into four great classes viz., 

 the Crustacea (crabs, lobsters, etc.), the Arachnida (mites, 

 spiders, and scorpions), the Myriapoda (centipedes and gally- 

 worms), and the Insecta (or true insects). These are roughly 

 distinguishable from one another by the following charac- 

 ters 



