CHAPTER V 



PHYLUM ARTHROPODA, CLASS CRUSTACEA, 

 LAW OF BIOGENESIS 



The animals belonging to the phylum Arthropoda are 

 readily distinguished from all others by the presence of 

 an exoskeleton and by the uniform occurrence of paired, 

 jointed appendages arranged metamerically. This is an 

 important phylum numerically for it contains more species, 

 than* all the rest of the animal kingdom 400,000 or more. 

 It includes the crayfishes, crabs, centipedes, insects, spiders, 

 ticks, scorpions, etc. 



In order to understand the general structure and rela- 

 tionships of the arthropods, examine Fig. 27. This shows 

 the structure of an hypothetical ancestor which may be 

 supposed to have given rise to all the existing groups of 

 arthropods. This imaginary animal was worm-like, meta- 

 meric, and had a tubular digestive system with a mouth 

 at one end and an anus at the other. There was a hard 

 exoskeleton, covering every segment of the body, and 

 metameric appendages. The appendages were rather 

 broad and flat; each consisted of a jointed basal piece and 

 two distal portions hence, was a biramous appendage. 



Anterior and posterior ends were well differentiated. 

 Several segments at the anterior end had grown together 

 to form a head which bore a pair of antennae, a pair of 

 eyes, and a pair of biting jaws or mandibles. A pair or 

 two of appendages, the maxillae, which were added to the 

 head when the metameres just behind fused with it, had 

 come to serve with the mandibles as " mouth parts" for 

 manipulating the food. At the posterior end of the body 

 the appendages had perhaps become specialized for purposes 

 of reproduction. 



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