OF MYRIAPODA AND MACROUROUS ARACIINIDA. 
293 
passes in this manner from the artery, on the dorsal surface around the colon, to unite 
beneath a ganglion of the cord with the subspinal vessel on the ventral surface ; thus 
forming a series of four vascular collars ( r.r.r . r.) around this portion of the alimen- 
tary canal, one in each segment, at its commencement nearest the abdomen. At the 
distal end of each segment a second set of vessels is given off, but these are given 
solely to the muscular and other structures, and do not form any anastomoses with 
the subspinal vessel. 
Besides the arteries already described, there are others which deserve to be regarded 
as proper visceral arteries. These originate from the inferior surface of the aorta 
before it spreads out on the oesophagus into great branches. They come off from 
the under surface of the aorta, either in pairs, as in the genus Buthus, Leach (fig. 33.x), 
or as a single trunk, as in Aadroctonus, Koch (fig. 32. c, d), as we have seen in the 
caudal vessels. They are given specially to the alimentary canal and the liver, to which 
they give off a branch on each side (/'), opposite to each junction of this viscus with 
the alimentary canal (d). They also anastomose extensively along the whole of the 
abdomen with branches from the proper systemic arteries, and are continuous with 
branches which pass forwards on the alimentary canal from the trunks of the systemic 
arteries from the great chamber of the heart in the sixth abdominal segment. The 
beautiful uniformity of the principles of development is thus further illustrated in 
these vessels. Like the great divisions of the aorta these also are modifications of 
the systemic arteries, the primary vessels of the segments, and are specially given to 
the alimentary canal at its anterior extremity, as the systemic arteries in the Myria- 
poda are to the hepatic appendages of the same structure at its sides and posterior 
extremit} r . 
The portal system of vessels (figs. 30, 31.) is situated chiefly below the nervous 
cord on the ventral surface of the body, and is the means by which the blood is col- 
lected and conveyed to the branchiae, from which it seems to be returned to the 
system, after circulating through the organs, by means of a large sinus or vessel at 
their posterior superior angles. Behind the bony arch of the thorax (fig. 31. b) there 
is a hollow fibrous structure (d) that closely surrounds the cord and nerves, as in a 
sheath, but the precise nature of which I have not fully ascertained. It seems to 
form a kind of sinus, from the posterior part of which a small vessel passes back- 
wards, which, joined by anastomoses from the supra-spinal artery, forms the com- 
mencement of the subspinal vessel ( g ) ; and it gives off two pairs of vessels at its sides. 
The first ( h ) and second (e) pair of these efferent vessels, covered by the thick peri- 
toneal lining of the abdomen (;?), send the blood in a diagonal direction backwards 
to the first pair of abdominal branchiae ( k ). The first pair of these vessels originate 
close to the folds of the diaphragm (a?). They pass backwards and outwards into the 
abdomen, and are joined in their course by numerous small vessels (/) from the sides 
of the segments ; and immediately anterior to the first pair of abdominal branchiae 
are each divided into two brandies (m), which are again divided and subdivided into 
2 Q 2 
