OF THE AMERICAN KING-CRAB. 483 
arising near the last pair of * ostia, passing obliquely outward and backward. The 
posterior or * pleonic’ artery (Pl. XXXVII. fig. 1, 7) 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, then sub- 
divides and blends with the sinuses continued from the ventral chord and investing the 
* eauda equina” of the tail-spine. 
The veins, or venous sinuses with the least indefinite form, are those that course along 
beneath the mediilateral ridges of the cephaletron in association with the arteries 
(Pl. XXXVI. fig. 2, », 2), and those which follow and lie near the margins of both 
cephal- and thorac-etra. The latter return their blood by the posterior veins (ib. 7), 
united by the median channel (s) with the pair in advance, q; their common trunk open- 
ing into the hind part of the pericardial sinus, 0’. 
$ 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 (1x-xtrr, 
Pls. XXXVII. and XXXVIIT.). 
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. XXXIX. 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. 
From a venous sinus along the base of attachment of the gill-plates * the blood passes 
and again subdivided opposite the second pair of cardiac valves” (Pl. XXXVI. fig. 2, c, f), ** one current following the. 
edge of the cephalothorax " (ib. n), “and the other going on towards the heart” (ib. 5). “The abdominal arteries, 
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." — Development of Limulus polyphemus, pl. y. fig. 27, p. 171. 
This admirable memoir appeared subsequently to the reading of the present paper before the Linnean Society; and 
the Report given in the Number of * Nature ” for January 25, 1872, is quoted by Dr. Packard at p. 201. Dr. Packard 
uotes that the heart * beats ninety times a minute," in the larva after the first moult. 
* « [] parait exister une libre communieation entre ces diverses poches respiratoires ; car, en introduisant de l'air 
dans une de ces duplicatures, on voit non seulement s'écarter les lames de la méme branchie, mais méme se. gonfler 
9T2 
