CHAPTER XVIII 

 PHYLUM ARTHROPODA 



Arthropoda (ar throp'O da, joint foot) is the name of the largest 

 known group of animals. As the name implies, all representatives 

 of the phylum have paired, jointed appendages and a definite 

 tendency toward specialization of them. Their bodies are triplo- 

 blastic, segmented, bilateral, and covered by a chitinous exoskele- 

 ton. The coelom is modified by a marked reduction as a result of 

 specialized vascular spaces. The segmentation or metamerism of 

 the body is expressed in a high degree in this phylum and there 

 is a definite relation of appendages to segments. The segments 

 have undergone greater specialization and greater regional differ- 

 entiation than was the case in annelids. In forms where there is 

 little or no differentiation of segments, the condition is referred to 

 as homonomous, while a highly differentiated condition of segments 

 as found in most arthropods is spoken of as heteronomous. This 

 group has fairly distinct head, thorax, and abdomen. The append- 

 ages on various segments are typically homologous with each other. 

 Some are modified as sense organs, others as mouth parts, others 

 for walking, swimming, and reproduction. 



The skeleton is entirely exoskeletal, composed of chitin, and fits 

 exactly the shape and contour of the body. Since it is fairly un- 

 yielding to growth, it becomes necessary for the arthropod to shed 

 the skeleton periodically during its growing periods. This molting 

 or ecdysis, as it is called, is quite characteristic of many of the divi- 

 sions of this phylum. 



The circulatory system is of the ojyen type, since there are large 

 sinuses or spaces surrounding most of the organs instead of a con- 

 tinuous circuit of blood vessels. The nervous system is of a modified 

 ladder type with a ventrally located cord. The digestive system 

 shows specialization in that it is divided into distinct regions as an 

 adaptation to special types of food which require mastication. 



Classification 



This phylum is divided into two sections and at least five classes ; 

 some authors recognize as many as eight. The sections are deter- 

 mined according to the means of respiration. 



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