ARTHROPODA 



351 



and are arranged in mctamcric muscle groups. Frequently they are inserted 

 on the chitin by special tendons, portions of the chitin drawn inwards. Through 

 such infoldings there arises in many arthropods an 'entoskeleton.' 



Another important character is the heteronomous segmentation, which, 

 in the lowest forms (Peripalus and myriapods), is little pronounced, but 

 elsewhere leads to a marlied inequahty of the regions of the body. A 

 few segments at the anterior end always fuse and form a head (fig. 365, C) ; 

 behind this there is usually a second segment complex, the thorax 

 {pcreioii) (T), and then a third, the abdomen {plcon) {A). An apparent 



Fig. 365. Fig. 366- 



Fig. 365. — Campodea staphylinus (from Hu.xley). A, abdomen; C, head; T, thorax. 

 Fig. 366. — Euscorpius italicus. a, abdomen; c, cephalothorax; p, post-abdomen; 

 s, sting; i, chelicerx, 2, pedipalpi; 3-6, walking legs. Below chelicera enlarged. 



reduction of regions occurs when the head and thorax unite (fig. 367, Ct) 

 to form a cephalothorax; or the number of regions may be increased (fig. 

 366) by a division of the abdomen into abdomen proper {a) and post- 

 abdomen {p). Finally, in many arthropods (e.g., the mites or acarina, 

 fig. 368) it is impossible to recognize regions or somites because internal 

 fusion of parts has obliterated the external evidences of segmentation. 

 In order clearly to understand what is meant by head, thorax, etc., 

 requires a consideration of the second character distinguishing the arthro- 

 pods from the annelids, the jointed appendages, which give the name to 

 the former group. The arthropodan appendages are highly developed 



