OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
259 
in Scolopendra, Llthobius, and Scutigera , they appear to come off behind the gan- 
glion, from the superior tract of the cord, to which they have been closely joined in 
these more perfect forms of the nervous system. This difference of position is readily 
explained by the changes which have been seen to take place in Polydesmus, and 
which are also in operation in Geophilus. In the posterior part of the cord, which 
is the part last formed during the gradual and successive changes of these animals, 
there is but a short space between these nerves, which lie above the aganglionic tract 
of the cord, and the commissural fibres of the nerves from the ganglia. But in the 
course of the development of the segments of the body by elongation, a correspond- 
ing elongation of the nervous cord itself takes place in the substance of the ganglia, 
and these nerves become further and further removed from each other in that part 
which lies on the cord above the middle of the ganglion, while they still remain at- 
tached to the other nerves at its sides, and thus form a kind of triangle (fig. 13. 80. c) 
above the aganglionic tract, as it passes over the ganglion. This elongation of the 
cords in the ganglion goes on, as in Polydesmus, until in the middle portion of the 
body these nerves have become released from those at the sides of the ganglion, and 
pass off from its anterior part. In the anterior third of the body they are directed 
forwards, but are still kept in contact with the ganglion by means of the lateral fibres 
of reinforcement, and also by a delicate set of fibres derived from the ganglion itself 
(fig. 14. c ). These facts seem further to show that the ganglia are the centres in which 
the growth and elongation of the cord always take place, and also that the nerves of 
the cord are usually first formed in connexion with ganglia, although they may be 
afterwards removed from them by further growth and changes. 
The compound structure of the cord is distinctly seen in Geophilus. The superior 
aganglionic tract is more clearly seen along the whole length of the cord, even while 
passing over a ganglion, than in any other genus. It is scarcely more than one-half 
of the width of the whole cord. The lateral fibres are also distinct, and may be 
readily seen bounding the roots of the nerves. 
The disposition of the terminal nerves of the cord is curious. Each ganglion gives 
its pedal nerves backwards to the legs in the next segment, so that the penultimate 
pair of legs are supplied from the ganglion of the antepenultimate segment, and 
the terminal styles, or anal appendages, are supplied by nerves from the preceding 
segment. By this arrangement it is proved that the anal styles are not supplied by 
the terminal portion of the cord, as is usually the case, since there are three ganglia, 
much smaller than these, posterior to them (figs. 13 and 14. 83. e a). The anterior of 
these caudal ganglia (c) supplies the anal valves and segments, and may be regarded 
as the anal ganglion. The next ( e ), which also gives nerves to the anus, is partly sup- 
plied to the rectum, and part to the outlets of the female organs, which in Chilopoda 
are always placed in the same relation to each other as in insects. The terminal 
ganglion (a) is situated in the middle line of the junction of the receptaculi seminis , 
between these and the rectum, and corresponds in situation to a ganglion on the 
2 M 
MDCCCXLIII. 
