OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
253 
outlets of the female organs of generation, and consequently anterior to the third 
segment of the thorax. From its posterior extremity the cord is continued back- 
wards, in the middle line, between the female organs, immediately behind which it 
gives off a pair of nerves to these organs, apparently from the structure of the cord 
itself, but in reality from an atrophied ganglion (e), which has almost entirely dis- 
appeared from this part of the cord, precisely as similar ganglia disappear in the 
changes of insects ; thus showing the constant tendency of the gangliated portions 
of the nervous cord to become united. 
The number of segments in Polydesmus complanatus, Leach (Plate XI. tig. 6.), is 
twenty-two, including the head and anal segment. The number of ganglia in the 
cord, separate and distinct from each other, is thirty-four, each of which supplies one 
pair of organs of locomotion. Besides these there are the united ganglia ( d . 1, 2.) 
which supply the manducatory organs and the first and second pairs of legs. The 
nerves from the atrophied fourth ganglion (e) above alluded to are given to the two 
ovipositors of the female, the analogues of a pair of organs of locomotion ; and the 
thirty-eighth (37, 38.) is itself a double ganglion that supplies nerves to the apodal 
antepenultimate, penultimate, and anal segments. 
The brain ( b ) in this family affords some interesting considerations. The two lobes 
are very small, pear-shaped, and developed on their under surface into very long anti 
slender crura, which join beneath the oesophagus with the great aggregation of gan- 
glia. Each of these lobes is rounded on the external side ; and the optic nerves and 
ganglia are entirely absent, there being externally no organ of vision. On the front 
of each lobe there is a small elongated ganglion for the antennal nerve, which passes 
directly into each of those organs (a). This is a remarkable condition of the brain 
in these Myriapodes, and a similar one has been described by Treviranus* in Geo- 
philus, although in that genus, as I shall presently show, the optic nerves are not en- 
tirely absent, as in the Polydesmidae. This fact is especially interesting in reference 
to the analogy that is believed to exist between these lobes of the brain and optic 
ganglia, and the corpora quadrigemina of Vertebrata, and seems to show that their 
office is more important than that of simple ganglia of any individual organ ; and that 
the ganglia of the optic nerves themselves are those by which impressions are received 
from the retina and transmitted to the middle snpra-oesophageal ganglia, the brain, 
the common sensorium of the whole nervous system. 
The distribution of the ganglia and nerves of the cord deserve particular attention. 
On entering the fourth segment the cord is somewhat elongated and passes between 
the double outlets of the female organs, immediately behind which it gives off the 
nerves to those organs from the atrophied ganglion. These nerves are exceedingly 
large, and ramify extensively over the muscles and distal portions of those retractile 
structures. Behind this atrophied ganglion the cord itself gives off a pair of nerves, 
which are distributed to the sides of the segment; after which it almost immediately 
* Vermischte Schriften Anatomischen und Physiologischen inhalts. Bremen, 1817. 
2 l 2 
