OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
2 63 
nally distinct parts, which has been carried to so great an extent in t lie ganglia of 
the cephalothorax of the Scorpion, has also taken place in the development of the 
ganglia and cord of each segment of the abdomen, in the union of all the primary 
trunks of nerves into a single pair in each segment; so that throughout the whole 
body each ganglion appears to give off only one set of nerves; and this condition 
exists even in the ganglia of the tail. 
The cord in the tail has four distinct ganglia, one at the commencement of each of 
the first four joints. The terminal ganglion gives off two pairs of nerves. The first 
pair ( t ), are the proper nerves of the joint, and the second, the terminal nerves ( v ), are 
the continuations backwards of the two halves of the cord. One of these main trunks 
passes backwards on each side of the colon, and the two ascending, one on each side, 
and winding inwards, meet above it in the middle line of the dorsal surface, at the 
posterior part of the fourth caudal joint. On entering the fifth joint each trunk gives 
off a large branch to the muscles (17.), and then passes backwards, one on each side 
of the strong tendons of the flexor muscles of the dorsal surface. On entering the 
fifth segment these trunks again give off a pair of branches (w), but do not form gan- 
glia. The two trunks, now reduced to small nerves, pass backwards in the middle 
line, side by side, and give off a few lateral branches to the muscles of the sixth joint, 
th e aculeated joint of the tail, through which they pass parallel to each (y) other 
along the dorsal surface to the extremity of the poison duct (z) and point of the sting. 
At the termination of the fifth segment they send a branch on each side of the anus, 
which is thus inclosed between them (x) as in an elongated nervous collar, and im- 
mediately behind it (18.) they give off the branches to the base of the sting without 
forming a ganglion. 
The nervous cord is narrowed and rounded in the abdomen, but in the tail it is 
thin and flattened, and spread out like a riband, and is larger in proportion to the 
size of the nerves, than in any other part of the body. Throughout its whole length 
the two halves of the cord are closely approximated, excepting immediately anterior 
and posterior to each ganglion, where the blood-vessels pass downwards from the 
supra-spinal artery to join the sub-spinal structures. The pair of nerves given off from 
each ganglion in the tail are divided into two sets of branches, and these are again 
subdivided. The anterior set (t) is the smallest, and is directed forwards and down- 
wards to the muscles of the under surface of the joint. The posterior set (u) is divided 
into four trunks, which pass upwards and backwards, and are given, like the lateral 
branches from the terminal ganglion, to the great flexor muscles, which pass down- 
wards on each side from their strong tendinous attachments in the preceding joint. 
The ganglia of the tail are oval, flat, and thin, like the cords, but those of the abdo- 
men are thick and elongated, oval or spindle-shaped, but flattened on the upper 
surface. The two nerves, which are united into a single trunk at the side of each 
ganglion, are derived, one from its anterior, and the other from its posterior border. 
The anterior one (PI. XV. fig. 38. a) passes backwards until it meets with the pos- 
