CHAPTER VIII 



STRUCTURE AND CLASSIFICATION OF JOINTED-LIMBED 

 ANIMALS (Arthropoda) 



This is by far the largest of the great groups of the animal 

 kingdom, including, as it does, Insects; Scorpions, Spiders, and 

 Mites; Centipedes and Millipedes; Lobsters, Crabs, Shrimps, and 

 a host of other Crustacea. 



The Lobster, a typical member of the phylum, has already 

 been briefly described in illustration of the characters of the 

 higher Invertebrates (see pp. 302-304). It may be convenient 

 to point out the respects in which it is typical of the group 

 Arthropoda. These are: bilateral symmetry, the division of the 

 body into a series of segments grouped into regions and bearing 

 a series of paired jointed lim.bs, and the presence of a central 

 nervous system consisting of a nerve-ring and a ventral cord. 

 It may also be noted that the so-called body -cavity consists of a 

 set of blood containing spaces situated between the internal 

 organs and the wall of the body. They form, therefore, a part 

 of the blood-system, while the body-cavity of a Vertebrate (see 

 p. 42) belongs to the lymph - system. Molluscs agree with 

 Arthropods in this respect, except that in them the pericardial 

 cavity does not contain blood, but is comparable to the corre- 

 sponding cavity in a Vertebrate so far as that particular feature 

 is concerned. In Arthropods, however, the heart is situated in 

 a blood-containing space from which blood passes into it. 



The phylum is divided into the following classes, which will be 

 considered in the same order: — 



A. — Air-breathing Forms (Tracheata). 



Class I. Insects (Insecta). 



Class 2. Spider-like Animals (Arachnida). — Scorpions, Spiders, Mites 



Class 3. Centipedes and Millipedes (Myriapoda). 



Class 4. Peripatus (Prototracheata). 



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