OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
301 
arteries are now first pointed out in the Myriapoda, and their analogues shown in 
vessels, which heretofore have been imperfectly traced by Treviranus in the Arach- 
nida. The course of the blood to the heart has hitherto been regarded as simply in- 
tercellular ; but the conclusion now arrived at, from a careful examination, is, that in 
the perfect state of the animal the blood always flows in vessels, the parietes of 
which are usually quite distinct, although in some parts of their course they have 
rather the character of sinuses. These vessels empty themselves at the valvular 
orifices of Straus, with which they appear to be in direct communication, and sur- 
round the orifices as delicate membranous structures. They convey back the blood 
from the branchiae in the Arachnida around the sides of the body, and are joined by 
other vessels in their course, and form numerous connexions with the adipose tissues ; 
so that some intermixture of venous and of recently aerated blood from the branchiae 
takes place in the course of the fluids to the heart. 
The structure of the heart and vessels is particularly examined in the Myriapoda 
and Arachnida, and the vascular collar in each, formed by the aortic arches, at the 
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anterior extremity of the aorta, is shown to be a modified condition of the systemic 
arteries. The distribution of vessels from these arches, both in the Myriapoda and 
Arachnida, is minutely traced, and the analogies pointed out between the seemingly 
confused and aggregated distribution in the Arachnida, with corresponding portions 
of the same structures in the more distinctly developed parts in the Myriapoda. This 
identification of parts of an intricate distribution of vessels in one animal, and their 
comparison with those well ascertained in another, exemplifies the uniformity of plan of 
creation, and the utility of comparative examinations of structure, even in the lowest 
forms of animals. The course of the supra-spinal artery, formed by union of the 
aortic arches around the oesophagus, is followed into the abdomen, to the extremity 
of the body in the Myriapoda, and to that of the caudal region in the Scorpion : and 
the distribution of its branches in these classes is traced outwards along the course 
of the nerves from the ganglia. Branches from the systemic arteries are traced to 
their ramifications on the coats of the hepatic vessels, in the first of these animals ; 
and others from those of the visceral systemic arteries to the alimentary canal and 
liver in the latter ; facts which I regard as new demonstrations of the existence of 
circulatory vessels distributed to the internal organs in Articulata. 
A set of vessels which I formerly described as portal vessels are also further exa- 
mined, and are traced to their terminations in extensive anastomoses and web-like 
capillary distributions on the branchiae. The structure of the branchiae, and the 
manner in which the blood seems to permeate their delicate laminae, and is after- 
wards collected in sinuses at their base, and again returned to vessels that convey it 
back to the heart, are also investigated; and the vessels specially given for the nourish- 
ment of the branchial structures are noticed. The course of the circulation is traced 
from the heart, through the systemic arteries, laterally, to the sides of the segments ; 
and forwards, through the chambers and the aortic arches into the cephalothorax 
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