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- 



, . , , , r . . , , . ., Fig. 43. NERVOUS SYSTEM OF 



testmal tube, i, ?., to terminate at the AN INSECT. 



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, c; and 

 along the under- side of the body is seen the chain of ganglia, 



Fig. 44. DIAGRAM SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN 

 THE ARTICULATA. 



g. 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, k, 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 



