CHAPTER XXII 

 THE CRAYFISH, A CRUSTACEAN ARTHROPOD 



Arthropoda (ar throp' 6 da, joint iootj is the name of the largest 

 known phylum of animals, and the crayfish is a member of this group. 

 As the name implies, all representatives of the phylum have paired, 

 jointed appendages and a definite tendency toward specialization of 

 them. Their bodies are triploblastic, segmented, bilateral, and covered 

 by a chitinous exoskeleton. The coelom is modified by a marked re- 

 duction 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 differentiation than was the case in annelids. In forms where 

 there is little or no differentiation of segments, the condition is re- 

 ferred to as homonomous, while a highly differentiated condition of 

 segments as found in most arthropods is spoken of as heteronoynous. 

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

 pendages 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 open 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 

 adai^tation 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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