26 



arising near the last pair of ' ostia,' passing obliquely outward and backward. The 

 posterior or ' pleonic ' artery (PI. II. fig. 1, t) has more definite tunics and holds a 

 longer course than those from the fore part and sides of the heart. It is wavy at its 

 beginning, in relation to the varying directions of the tail-spine in its flexile movements 

 upon the body. The artery having entered the body of the spine, continues its course, 

 as such, along the dorsal side of the cavity, through two thirds of its length, hten sub- 

 divides and blends with the sinuses continued from the ventral chord and investing the 

 ' Cauda equina ' of the tail-spine. 



The veins, or venous sinuses with the least indefinite form, are those that course along 

 beneath the medi-lateral ridges of the cephaletron in association ^vith the arteries 

 (PI. I. fig. 2, H, u), and those whicli follow and lie near the margins of both cephal- 

 and thorac-etra. The latter return their blood by the posterior veins (ib. r), united by 

 the median channel (s) with the pair in advance, q ; their common trunk opening into the 

 hind part of the pericardial sinus, b'. 



§ 7. Respiratory System. — The gills consist of thin membranous plates of a broad semi- 

 oval shape ; there are from 150 to 200 in each gill or group, the number diminishing in 

 the hinder ones. The gills are in pairs, attached to the upper, hinder, or inner surface 

 of the proximal joints or broad coalesced plates of the last five thoracetral limbs (ix-xiii, 

 Pis. II., II. A, III. and IV.). 



The branchial plates overlap each other from before backward. The anterior and 

 exterior one is the smallest ; the others progressively increase to a little beyond the 

 middle of the series ; the hindmost again diminish, but in a less degree ; the whole 

 mass has the full oblong or irregular oval form shown in fig. 2, PL V. Each plate is 

 strengthened by a chitinous filament along its free border, thickest where this is exposed, 

 so that the length of the gill is greater at its free or floating side than along its attached 

 base : the free margin is also ciliate. 



Each gill-plate consists of two layers or membranes, united along the chitinous border, 

 and also by numerous filaments so far apart as to divide the interspace into reticular 

 canals or cells, smallest at a subcentral space (fig. 3, a), and affecting a concentric 

 arrangement as they approach the free borders of the gill-plate. The two constituent 

 layers of the branchial plate may be regarded as productions or duplicatures of the 

 delicate skin of the upper or inner surface of the lamelliform limb. 



Fi'om a venous sinus along the base of attachment of the gill-plates * the blood passes 



represented by powerful currents of blood issuing from between the last two pairs of cardiac valves, are directed 

 obliquely outwards and backwards. The caudal aorta sends a current nearly to the tip of the spine, the venous 

 sinuses returning it along the sides. The simple arrows mark the course of the returning currents, which flow from 

 all parts of the body towards the valves." — Devehpment o/Limulus polyphemus, pi. v. fig. 27, p. 171. 



This admirable memoir appeared subsequently to the reading of my paper ' On Limulus ' before the Linnean Society ; 

 and the Report given in the Number of ' Nature ' for January 2.5, 1872, is quoted by Dr. Packard at p. 201. Dr. 

 Packard notes that the heart " beats ninety times a minute," in the larva after the first moult. 



• " II parait exister une libre communication entre ces divcrses poches respiratoires ; car, en introduisant de Fair 

 dans une de ces duplicatures, on voit nou seulement s'ecarter les lames de la memo branchie, mais meme se gonfler 

 toutes les branchies, ainsi que I'espace membraneux entre les pattes abdominales."' — V. der Hoeven, op. cit. p. 19. 

 The intercommunicating passage is the basal sinus, related to the gills, physiologically, as a ' branchial artery.' 



