OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
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penetrate the substance of the ganglion accompanied by the arteries given to it; from 
the lateral branches from the spinal artery. 
The small median cephalic artery (fig. 23. a. q ), given off from the heart with the 
aortic arches to the head, passes along the oesophagus to the posterior pair of muscles 
of the pharynx, behind which it gives off its second pair of branches. These pass 
downwards to the maxillae and internal parts of the mouth and unite beneath the 
pharynx, but do not send any vessel backwards to join the great spinal artery at its 
commencement and junction with the aortic arches, as in the Scolopendra. The 
small median trunk then passes beneath the brain, and is divided on the front of 
it into two pairs of branches, one of which passes laterally to the organs of vision, 
and the other proceeds forwards on the inner side of the great nervous trunk to the 
antenna. I have not yet ascertained to what distance this vessel is extended in the 
antenna of the perfect individual, whether throughout the greater part of the organ, 
forming a series of loops in the joints, or whether the whole trunk is returned back- 
wards across the third joint of the antenna, in the course of the current of the blood 
seen in this part of the head in the young Lithobius. It is quite certain that the 
course of the blood backwards in the antennae and head is in a vessel on the external 
side of the antenna, both in Lithobius and Scolopendra. The existence of this vessel 
I have repeatedly traced in Scolopendra, backwards from the antennae, beneath the 
optic ganglia, to its union with a small median trunk beneath the oesophagus that 
passes backwards between the nervous cords to the junction of the aortic arches and 
spinal vessel. The course of the blood in the antenna in the young Lithobius is 
always forwards on the inner side, and backwards on the outer side of the organ. 
In the Scutigeridce the circulatory system affords a still further proof of the prin- 
ciples already advanced, that the complete development of every structure is by the 
union of two or more original parts. In the Iulidae we have already seen this very 
principle illustrated in the same animal, in different stages of its growth, in the union 
of two chambers of the heart in each moveable double segment ; and there is a 
further illustration of it in the permanent structure of the heart of the Scutigeridse. In 
this family the number of chambers is still fifteen, as in Lithobius, but every alternate 
chamber (fig. 25.) is reduced both in size and extent. This reduction of length had 
already begun to take place in Lithobius, in which the dorsal plates of the body are 
alternately longer and shorter in the different segments, to the respective lengths 
of which the chambers of the heart were begun to be reduced. This is a condition 
preparatory to the union in pairs, first of the dorsal plates, and afterwards of the 
chambers of the heart. In the Scutigeridse the dorsal plates are already united, and 
form but eight moveable coverings, one to each pair of segments, which still remain 
distinct on the ventral surface of the body. But although there are still sixteen 
chambers to the heart, the changes commenced in Lithobius are carried still further 
in this genus, and each alternate chamber is very much smaller and shorter than 
the one next before it, and covered by the same dorsal plate. But although the 
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