206 BRITISH FOSSIL CRUSTACEA. 



corresponding with the second osteal vein. This vein courses outward near the hind 

 border of the cephaletron, bends forward, and runs parallel with the lateral or ocular 

 ridge as far as the compound eye : its branches are short, and speedily expand into 

 sinuses. 1 A pair of arteries are obscurely indicated arising near the last pair of ' ostia,' 

 passing obliquely outward and backward. The posterior or pleonic artery (PI. XXXIV, 

 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, then subdivides 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 medilateral ridges of the cephaletron in association with the arteries, 

 and those which follow and lie near the margins of both cephaletra and thoracetra. The 

 latter return their blood by the posterior veins, united by the median channel with the 

 pair in advance, their common trunk opening into the hind part of the pericardial sinus. 



§ 5. — " 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 — xin, 

 Pis. XXXIV and XXXV). 



" 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 



1 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 couid 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 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 

 under the ocelli, and another along the outer edge of the cephalic shield, and again subdivided opposite 

 the second pair of cardiac valves, one current following the edge of the cephalo-thorax and the other going 

 going on towards the heart. 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 polyphenols? pi. v, fig. 27, p. 171." This admirable 

 memoir appeared subsequently to the reading of Prof. Owen's paper before the Linnean Society, and the 

 report given of it in the number of 'Nature' for January 25th, 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. 



