CHAPTER XXXV. 



ARTHROPODA CRUSTACEA, ARACHNIDA, AND MYRIAPODA. 



THE Division Arthropods, includes a multitude of animal 

 forms which are distinguished from those already considered by 

 the possession of special organs of locomotion, by which they 

 may swim, creep, run, climb or fly. They are for the most part 

 terrestrial or aerial animals. With the organs of locomotion a 

 solid support was necessary for the attachment of the muscles, 

 hence the outer layer of the skin is hardened into a firm exo- 

 skeleton. 



The body among the arthropoda is divided into three parts, 

 the head, thorax and abdomen, the appendages of each having 

 different structure and function. Each of these parts seems made 

 up of segments. The head is covered by a hard integument, in- 

 closes the brain and bears the sense organs and mouth parts. 

 The appendages of this region are changed so as to form antennae 

 and jaws. The thorax is a fusion of seven segments having a 

 hard integument. In some cases the head and thorax are fused 

 together forming the cephalothorax. The thorax bears the 

 appendages most important for purposes of locomotion. The 

 abdomen, composed of distinct rings or segments, seldom has 

 appendages, but when present they aid in locomotion, or in res- 

 piration, or may serve for carrying the eggs. 



The skin is of two layers, a soft growing layer and an outer 

 one that has been hardened by the deposition of calcareous mat- 

 ter in the chitin base, so as to form a fi^m external skeleton. The 

 hairs, setae, spines, bristles and ot' -r appendages of the skin, 

 have their origin in the deeper layer. During the period of growth 

 this shell is cast off and renewed from time to time when the ani- 

 mal is said to moult. The muscles are all of the cross-striped 

 variety. 



(267) 



