482 PROF. OWEN ON THE ANATOMY 
by the seeming abdominal arterial trunk continued backward from the neur-arterial 
circle (Pl. XXXIX. a, B, to or. of nerve 219). On slitting open the coats of the blood- 
vessel, and washing out the flake of injection, the ganglionie nervous chord was exposed 
in its interior *. The same result followed the like perquisition of the smaller ramifica- 
tions of the vascular system into which the injection had penetrated, and engendered the 
conviction that the main pair of arteries had but a brief course as such f, becoming 
resolved, on reaching the neural ring, into blood-sinuses—a condition which prevails 
throughout a great proportion of the vascular system of Limulus. The whole nervous 
system, save where the terminal twigs are lost in the tissues, is bathed in the blood of 
these sinuses, which retain the appearance of ramified vessels, through their relations 
to the nerves as the vascular envelopes of these {. Elsewhere the sinuses expand, lose 
the character of tubes, or vessels, oceupy the interspaces of viscera and muscles, initiate 
the ramified branchial system of vessels, and finally return the blood to the pericardial- 
like sinus enclosing the heart. 
A pair of arteries is sent off near hia, anterior pair of ostia, and are closely connected 
with the much larger veins emptying the neighbouring sinus into the corresponding 
parts of the pericardial one. These arteries (Pl. XX XVI. fig. 2, ^) pass outward and 
forward, and subdivide into branches, which are lost upon the epimeral nerves. I was 
unable to distinguish an artery corresponding with the second ostial vein (ib. f^. This vein 
courses outward near the hind border of the cephaletron, bends forward at », and runs 
parallel with the lateral or ocular ridge as far as the compound eye: its branches are 
short, and speedily expand into sinuses $. A pair of arteries are obscurely indicated, 
* The neurine thus seems to be small in proportion to the thick neurilemma, as Gegenbaur remarks ; but he did 
not recognize the share taken by the arterial tissues i in this sheath :—“ Bezüglich des feineren Baues soll die schon 
oben angeführte dicke Umhüllung des Schlundringes erwühnt werden, derzufolge der eigentliche Nerventheil des 
Schlundringes relativ klein erscheint." Op. cit. p. 241. 
+ They are shown as cut off from the arches and lost upon the brain in Pl. XXXIX. 
t This interesting stage in the differentiation of nerves and vessels was demonstrated in my Hunterian Lectures of 
1852, * Organization of the Entomostraca illustrated in the Limulus, Lecture xvi. Crustacea, *Synopsis, March, 
1852, and is briefly enunciated in the volume on INVERTEBRATA as follows:—** The sides of the great esophageal 
ring are united by two transverse commissural bands: but the most remarkable feature of the nervous axis of this 
Crustacean is its envelopment by an arterial trunk. A pair of aortæ from the fore part of the heart arch over each 
side of the stomach, and seem to terminate by intimately blending with the sides of the œsophageal nervous ring. 
They, in fact, expand upon and seem to form its neurilemma; a fine injection thrown into them coats the whole 
central mass of the nervous system with its red colour."— Lectures on the Comparative Anatomy and Physiology of the 
Invertebrate Animals, by Prof. Owen, F.R.S. (second edition, London, 1855, Lecture xvi. p. 310). Gegenbaur, in 
his histologieal treatise on Limulus (op. cit. p. 241), remarks:—* Auch die peripherischen Nerven sind sümmtlich 
von einer dicken Hülle umgeben, die sogar noch makroskopisch erkennbar ist." 
$ That an arterial canal accompanies the vein is indicated by the course of the blood, as observed by Packard in a 
living larva of Limulus :— I could not see the walls of any of the arteries; and indeed the arterial blood seemed to 
flow in channels exactly like the venous sinuses, as in the arteries which pass around the margin of the carapace the 
blood-disks were seen to pass by irregular currents towards the front edge of the margin. The anterior aorta 
could not be detected in the young Limulus; but on each side of the end of the heart the blood could be seen 
rushing out and in, and with a general course downwards, beneath the cesophagus, while a current of blood flowed 
on each side of the stomach and cesophagus, and thence went out at a considerable angle to the edge of the carapace, 
where it divided, sending a branch around under the ocelli, and another along the outer edge of the cephalic shield, 
