226 



NATUBAL HISTOET. 



382. I have spoken in § 15 of the ganglia 

 of the nervous system of the Articulates. 

 Commonly there is a chain of them, as seen in 

 Fig. 8. But sometimes, as in some of the 

 Crab tribe, there are only two, one in the head 

 and the other in the thorax. This is a de- 

 cided approach toward the arrangement of 

 the nervous system of the Vertebrates, the 

 upper ganglion being somewhat like a true 

 brain. 



383. The muscles constitute the bulk of 

 the body in the Articulates ; and, being thus 

 muscular, they are very active. For this pur- 

 pose the armor-skeleton is made as light as 

 possible consistently with the requisite firm- 

 ness. The movements of some of them ex- 

 ceed in rapidity those of any other animals. 

 With the exception of one group, they roam 

 freely in search of their food, and have very 

 effective organs for capturing their prey. 

 Their apparatus for mastication, also, is com- 

 monly complicated and powerful. 



384. Almost all of the Articulata have a distinct head. 

 The jaws do not move up and down as in the Verte- 

 brates, but sidewise. There are often several pairs of 

 them, sometimes having cutting edges, sometimes edges 

 with saw-like teeth, and sometimes they are fitted to 

 crush rather than to cut or tear. The legs are gener- 

 ally six, eight, ten, or fourteen in number, but some- 

 times there are many hundreds. Sometimes there are 

 none ; but when they exist at all, they are never less than 

 six. 



885. The circulation of the Articulates is peculiar. 

 There is no heart, but instead there is a tube stretching 

 along the back. This is not perfectly regular, but has 

 segments corresponding with the segments of the body 

 mentioned in § 881. Each segment is a sort of heart for 



