398 
Avgas persicus 
mentioned dorsal prominences lying immediately anterior to the efferent 
portion of the oesophagus. These are the true cerebral ganglia, which 
in the Ixodid ticks give rise to the optic nerves, and the nerves of the 
porose areas, in those cases where such organs are present; in Argas, 
however, these sensory organs are absent, and apparently no nerves are 
derived from the cerebral ganglia. 
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The cheliceral ganglia, which form the anterior portion of the 
“ supraoesophageal ganglion,” are situated immediately below and in 
front of the cerebral ganglia, and from the antero-dorsal face of each 
the cheliceral nerve issues. A stout bundle of nerve fibres forms a 
connective, which descends from each of the cheliceral ganglia, the two 
connectives passing down on either side of the oesophagus and forming 
a connection between the cheliceral ganglia and the suboesophageal 
portion of the brain 1 . 
The suboesophageal portion forms the greater part of the brain, and, 
as already stated, all the remaining nerves originate from this part. 
The first pair of ganglia—the palpal ganglia —are situated in the 
antero-lateral region, on either side of the place of entry of the oeso¬ 
phagus, and give rise to the nerves which innervate the palp. The 
entire lateral region of the suboesophageal portion of the brain is 
divided into four successive ganglia, the pedal ganglia, from each of 
which a pedal nerve takes its origin. The posterior part of the brain, 
lying behind and between the fourth pair of pedal ganglia, is seen to 
consist of four small splanchnic ganglia, disposed in pairs on either side 
of the median line, from which the splanchnic nerves derive their origin. 
The nerve cells which constitute the peripheral ganglionic layer of 
the brain are all of the unipolar variety (see Text-fig. 7). We have 
succeeded in identifying, in Argas persicus, two of the three different 
types which Nordenskiold found to exist in the brain of Ixodes ricinus 2 . 
The larger ganglion cells are, for the most part, confined to the dorsal 
portion of the brain, immediately posterior to the place of exit of the 
oesophagus. A few large cells are, however, scattered among the 
ganglion cells in the ventral portion of the brain. All the remaining 
1 The origin of the cheliceral nerves from the supraoesophageal portion of the brain is 
apparently secondary. Wagner observed that in the developing larva of Ixodes calcaratus 
Bir. the cheliceral ganglia appear as the first of the series of six pairs of thoracic ganglia, 
and are then situated below the oesophagus, but in the subsequent process of condensation, 
they ascend to the supraoesophageal position and fuse with the cerebral ganglia. It may 
be remarked that the palpal ganglia are also involved in a lesser degree, as is indicated 
by. the position on either side of the oesophagus which they ultimately attain. 
2 Nordenskiold, E. (1909 c), p. 462. 
