GENERAL STRUCTURE OF ARTICULATA. 103 
and the ganglia themselves are composed of two halves, 
which have little connexion with each other. The chain 
thus formed (fig. 43) passes along the 
under-side of the trunk of the animal 
(as seen at g, fig. 44); not on what 
seems its back; and by the presence 
of this double chain of ganglia an 
Articulated animal may be distin- 
guished, even when, in its general 
structure, it should seem to belong to 
the group of Mollusca (§ 102). 
95. The general arrangement of the 
organs in the Articulata is shown in 
the accompanying figure of a Cray- 
fish. The mouth, situated on a pro- 
jecting head, opens into s, the stomach, 
from which passes backwards the in- Oo ae Se 
' testinal tube, ¢, 7, to terminate at the ~~" ax Insecr. 
opposite extremity of the body. The 
upper part of the tube is surrounded by the liver, £ which is 
here very large. In the head are seen the ganglia, ¢; and 
along the under-side of the body is seen the chain of gangli 
Fig. 44.—DIAGRAM SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN 
THE ARTICULATA, 
The blood is nearly colourless, and is usually impelled 
through the body not by a single organ or heart, but by a 
succession of contractile cavities, one for each segment, which 
open into one other longitudinally, forming what is known as 
the dorsal vessel ; in the Cray-fish and its allies, however, one 
part of this, A, is specially enlarged, so as in great degree 
to serve as a heart for the system generally. The respiratory 
organs are not connected with the mouth; and are not usually 
