CHAPTER XII 



PHYLUM ARTHROPODA 



ONE of the most striking features of the Annelida is the fact 

 that they are segmented, that is to say their body is divided into a 

 number of similar parts placed one behind the other like coaches in 

 a train, each of which to a greater or less extent resembles the part 

 in front of it. The likeness of the parts to one another varies. In 

 some worms we might easily detect from which region of the body any 

 given segment was taken. In the Earthworm we found, that, except 

 in the region of the clitellum, there is little external difference; 

 nevertheless if we consider the internal organs we can distinguish 

 any of the first twenty segments from any other behind these and 

 can easily arrange them in their proper order ; but no matter how 

 long the worm is, all the segments behind the twentieth resemble 

 one another so closely that it is impossible to assign any to their 

 right place, except the last of all (v. p. 142). 



The animals included in the group of the Arthropoda are 



segmented like the Annelida, but with few exceptions 



the number of segments is small and does not exceed 



twenty. The segments have also become more highly differentiated 



from one another in consequence of being modified to perform 



various functions, and they are more frequently fused together than 



is the case in the Annelids. 



The Arthropoda have jointed outgrowths called limbs or ap- 

 pendages. These are always arranged in pairs, and almost always 

 at least one pair is modified so as to assist in holding and crushing 

 the food. These modified limbs are termed gnathites (Gr. -yvdOos, 

 jaw). This character of possessing jointed limbs is what is indicated 

 by the name Arthropoda (Gr. apOpov, joint ; TTOV?, foot). 



The strength of these outgrowths lies in the thickness of the 

 cuticle secreted by the ectoderm surrounding them. The thickness 

 of the cuticle which constitutes the exoskeleton is the distin- 

 guishing feature of all Arthropoda. 



