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vessel, and washing out tlie flake of injection, the ganglionic nervous chord was expose: 

 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 hut a brief course as such t, 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 cbaracter of tubes, or vessels, occupy the interspaces of viscera and muscles, initiate 

 from a subcardiac sinus, the ramified branchial system of vessels, and return the blood 

 from all parts to the pericardial-like sinus enclosing the heart. 



A pair of arteries is sent off near the 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 (PI. I. fig. 2,/') pass outward and forward, 

 and subdivide into branches, which are lost upon the epimeral nerves. The next pau" of 

 arteries correspond with the second ostial vein (ih. /') : I was unable to trace them far. 

 This vein courses outward near the hind border of the ce]Dhaletron, bends forward at n, 

 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 iieurine tlius 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 in this sheath : — " Beziiglich des feineren Baues soil die schon 

 oben angefiihrte dicke Umhiillung des Suhluudringes erwahnt werden, dcrzufolge der eigentliche Nerventheil des 

 Schlundringes relativ klein erscheint." Ojp. cit. p. 241. 



t They are shown as cut off from the arches and lost upon the brain in PI. V. 



J Tins 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 Invertebkata as follows : — " The sides of the great oesophageal 

 ring are united by 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 aortte from the fore part of the heart arch over each 

 aide of the stomach, and seem to terminate by intimately blending with the sides of the oesophageal 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). A similar con- 

 dition, requiring injection for distinguishing the vessel from the nerve, is pointed out in the Scorpion {op. cit. p. 449). 

 Gegenbaur, in his histological treatise on Limulus (op. cit. p. 241), remarks: — " Auch die peripherischen Nerven 

 sind siimmtlich von ciner dicken Hiille umgeben, die sogar noch makroskopisch erkennbar ist." 



§ That an arterial canal accompanies the vein is indicated by tlie 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 Litnulus ; 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 oesophagus, while a current of blood flowed 

 on each side of the stomach and oesophagus, 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, 

 and again subdivided opposite the second pair of cardiac valves " (PL I, fig. 2, c, f), " one current following the 

 edge of the cephalothorax '" (ib. n), " and the other going on towards the heart " (ib. 6'). " The abdominal arteries, 



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