IXTEGUMEXT. ICERYOUS SYSTEM. 407 



region of the body called the cephalothorax (fig. 327). The thorax 

 bears the appendages which are of most importance in locomotion. 



The posterior portion of the body, or abdomen, is composed of 

 distinctly separate rings, and is, as a rule, without appendages. 

 When the latter are present, they serve partly as aids to locomotion 

 (abdominal feet), partly for respiration, or for carrying the eggs and 

 for copulation. More rarely, as for example in the scorpions, the 

 abdomen is divided into a broad anterior region, the prceabdomen, 

 and a narrow movable posterior region, the postabdomen. 



The skin, as in the Annelida, consists of two different layers, an 

 external firm, usually homogeneous chitinous layer, and an internal 

 soft layer, which is composed of polygonal cells (matrix, hypodermis) 

 and secretes in layers the at first soft chitinous cuticle (fig. 22). 

 The latter usually becomes hardened by the deposition of calcareous 

 salts in the chitinous basis, so as to form the firm exoskeletal 

 armour, which, however, is interrupted between each segment by 

 thin connecting membranes. The various cuticular appendages of 

 the skin (fig. 22, a, b, c), which may have the form of simple or 

 pennate hairs, of filaments, setae, spines and hooks, originate as 

 processes and outgrowths of the cellular matrix. The chitinous 

 cuticle together with its appendages is from time to time, principally 

 in the young stage during the period of growth, renewed, the old 

 cuticle being cast off as a continuous membrane (ecdysis, or moult). 



The muscular system never constitutes a continuous envelope, 

 but the muscles are usually broken up into segments which corre- 

 spond with the segmentation of the animal. The muscles of the 

 body are arranged in longitudinal and transverse bundles in the 

 different segments, and are frequently interrupted. There are in 

 addition large groups of muscles, which move the appendages. The 

 muscular fibres are always cross-striped. 



The internal organization is allied to that of the Annelida, but 

 does not present such a well-marked internal segmentation. 



The nervous system consists of brain, oesophageal commissures 

 and a ventral cord. The latter usually has the form of a ganglionic 

 chain (fig. 328), and is placed beneath the alimentary canal. * Some- 

 times, however, it exhibits great concentration, and may have the 

 form of an unsegmented ganglionic mass beneath the oesophagus. 

 The segmentation of the ventral ganglionic chain presents in details 

 the greatest variations ; in general, however, it corresponds to the 

 heterononious segmentation of the animal, in that in the larger 

 resions of the body, which have arisen by fusion of several segments, 



