226 NATURAL HISTORY. 
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 
is Fig. 8. But sometimes, as in some of the 
am/,, Crab tribe, there are only two, one in the head 
stigeo’” and the other in the thorax. This is a de- 
cided approach toward the arrangement of 
AT ~\ the nervous system of the Vertebrates, the 
A me upper ganglion being somewhat like a true 
Al Qi Prain. : 
Allis 383. The muscles constitute the bulk of 
wills» the body in the Articulates; and, being thus 
Ath. muscular, they are very active. For this pur- 
as “a pose the armor-skeleton is made as light as 
en \ possible consistently with the requisite firm- 
psa ness. The movements of some of them ex- 
‘((s} = ceed in rapidity those of any other animals. 
Ai" With the exception of one group, they roam 
4h NX freely in search of their food, and have very 
a> diy effective organs for capturing their prey. 
Fig. 178.—Cen- * S E . C e 
S pede. LE heir 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. 
385. 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 § 381. Each segment is a sort of heart for 
