BRANCH ARTHROPODA: CRUSTACEANS 145 



muscles. It may be flexible as in the sutures between 

 the body-segments in most insects, or hard and rigid as 

 in the sclerites of the segments. The firmness is due 

 primarily, and in the insects usually solely, to a deposit in 

 the cuticle of chitin, a substance probably secreted by the 

 underlying cells of the true skin, or it maybe due chiefly, 

 as in the crabs, to a calcareous deposit. In such cases it 

 becomes a veritable armor. The internal organs of the 

 Arthropods show a more or less obvious segmentation 

 corresponding with the segmentation of the body- wall. 

 The alimentary canal runs longitudinally through the 

 center of the body from mouth to anal opening. The 

 nervous system consists of a brain lying above the 

 oesophagus and a double nerve-chain running backward 

 from beneath the oesophagus, along the median line of 

 the ventral wall, to the posterior extremity of the body. 

 This ventral nerve-chain consists of a pair of longitudinal 

 commissures or cords and a series of pairs of ganglia, 

 arranged segmentally. The two ganglia of each pair are 

 fused more or less nearly completely to form a single 

 ganglion, and the nerve-cords are partially fused, or at 

 least lie close together. In addition there is a smaller 

 sympathetic system composed of a few small ganglia and 

 certain nerves running from them to the viscera, this sys- 

 tem being connected with the main or central nervous 

 system. In this group the organs of special sense reach 

 for the first time a high stage of development. Com- 

 pound eyes are peculiar to Arthropoda. The heart lies 

 above the alimentary canal. Respiration is carried on by 

 gills in the aquatic forms, and by a remarkable system of 

 air-tubes or tracheae in the land forms (insects). The 

 sexes are usually distinct, and reproduction is almost uni- 

 versally sexual. Most of the species lay eggs. 



The Arthropods are animals of a high degree of organ- 

 ization. The extremely diverse life-habits of the various 



