OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
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two segments, which are anchylosed together from a very early period of growth ; 
and that as the segments in the anterior part of the body become more and more 
nearly approximated, the gangliated portions of the nervous cord in those segments 
also become more closely united. Now what occurs in this respect in the nervous 
system takes place also in the vascular. Each double segment of the body in the 
Iulidee contains, at an early period of growth, two distinct chambers of the heart, 
each giving off its pair of arterial vessels, and furnished also with its two pairs of 
lateral muscles. I have found these chambers distinct, and still separated in lulus 
terrestris, so late in life as that which X shall hereafter have occasion to describe as 
the ninth period of development , when the individual possesses forty-four moveable 
segments. After that period the two chambers in each double segment unite and 
form but one chamber, while the reduplication of the muscular tunics, which form 
the boundaries of each double chamber, and in which are situated the auricular 
openings, becomes more complete. Each chamber of the heart, in the adult Xulidse, 
has therefore two pairs of systemic arteries, and four pairs of lateral muscles. The 
union of the two chambers seems to be occasioned by the growth and changes in- 
duced in the external coverings of the body, at the period when the animal undergoes 
its semi-metamorphosis, or change of tegument. 
The abdominal portion of the vascular system is less perfectly developed in the 
Chilognatha than the dorsal. The great ventral vessel, formed by the union of the 
aortic arches, is a wide dilated structure, that covers the upper surface of the 
nervous cord, and also the roots of the nerves to some distance from the sides of the 
cord (PI. XI. fig. 3. c). Certain processes are extended along the nerves to a short 
distance, and then appear to separate, and to form for them a vascular sheath (?‘) ; 
but this, in reality, is only on their upper surface, since I have distinctly observed 
vessels passing off from these structures, which I have traced to a great distance 
along the trunks of the nerves, as will presently be shown in the Chilopoda. I have 
seen these vessels very distinctly in the Spirostrepti. The upper surface of the cord 
in these families is thus covered by a vascular structure, at least three times as broad 
as the cord itself and its ganglions. I have reason to think that this structure is in 
reality formed by two vessels, one on each side of the cord, but connected trans- 
versely by a membrane that covers the cord; and that, consequently, in these families 
the blood is sent backwards in a double stream, from the sides of which vessels pass 
off to the sides of the body in the form of a vast number of minute trunks, and not 
in two or three principal trunks, as we shall find in the Chilopoda. This condition 
of the abdominal vessels, closely resembling that of the Annelida, is indicatory of the 
principle on which all large vascular trunks are originally developed, by the forma- 
tion, first of numerous minute branches, which anastomose, and are aggregated 
together in pairs, to form larger vessels. It is precisely similar in principle to the 
approximation of the ganglia of the cord to form enlarged portions of that structure, 
and to the lateral approximation of small branches of nerves in the formation of the 
principal nervous trunks of the body. 
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