CHAPTER 



15 



JOINT-FOOTED ANIMALS AND 



The Phyla Arthropoda 

 and Onychophora 



The Phylum Arthropoda 



Like the Annelida, the Arthropoda are bilateral, triploblastic, segmented 

 animals. Unlike the annelids, however, the arthropods have a much reduced 

 coelom, and the cuticle which covers the body in annelids is represented in 

 arthropods by a hardened exoskeleton which must be periodically shed to per- 

 mit growth. The name Arthropoda, which means "joint-footed," refers to 

 another conspicuous feature, the fact that the appendages are composed of 

 several divisions, so hinged together as to be capable of specific movements. 

 The taxonomy of arthropods is difficult and necessarily complicated, owing to 

 the relatively large number of difTcrent types of animals which possess the 

 general characteristics of the phylum but show important distinctions indi- 

 cating varying degrees of interrelationship. The scheme we shall follow, 

 although it is not sufficiently detailed to satisfy a specialist, will nevertheless 

 take into account many of these distinctions. 



We shall consider the phylum Arthropoda to be composed of three sub- 

 phyla: the subphylum Trilobitomorpha, the subphylum Chelicerata, and the 

 subphylum Mandibulata. The first of these is a group of extinct arthropods, 

 represented by the Trilobita, which had a long evolutionary history dating 

 from Pre-Cambrian times but are now known only as fossils. The members 

 of the subphylum Chelicerata are set apart from other arthropods because 

 they are without antennae, or specialized sensory outgrowths from the head, 



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