LIMULUS AN ARACHNID, 619 
of the most anterior pair of valvular apertures, a pair of 
lateral arteries is also given off in both hearts. 
The eight chambers of the Scorpion’s heart are placed in 
the seven anterior abdominal segments, the first correspond- 
ing to the segment which bears the genital operculum, the 
last two being placed in one segment (thirteenth of the 
whole series), the broad triangular segment which precedes 
the first cylindrical caudal segment. 
The eight chambers of the King Crab’s heart have a 
similar relation, though not so obvious. The anterior por- 
tion of the heart is somewhat drawn forward, so that the 
segments indicated by the valvular apertures are (like the 
corresponding nerve ganglion of the genital operculum) a 
good deal shifted to the front of the appendicular portions of 
the skeleton to which they are segmentally related. 
In place of the five hinder pairs of lateral arteries present 
in Scorpio, we find in Limulus large lateral arteries (fig. 18 7), 
which take origin by an anastomosis from the three pairs of 
anterior lateral arteries of the heart, and from the pair of 
lateral arteries of the base of the truncus arteriosus. 
The truncus arteriosus (or anterior portion of the heart, 
as M. Milne-Edwards prefers to call it) presents a remark- 
able agreement in the two cases in regard to the distribution 
and character of the vessels given off from it, although upon 
the basis of a fundamental agreement very wide differences 
in detail are to be noted. At the base of the truncus, just 
in front of the most anterior pair of valvular apertures of the 
heart, we have the pair of lateral arteries similar to those 
given off from the heart. Then the trunk is continued 
forwards (through the cephalothoracic region in Scorpio), 
and gives off two branches, which form a small vascular 
collar around the cesophagus in the Scorpion, but a wide 
pair of arterial commissures in the King Crab, which meet 
upon the postcesophageal portion of the nerve collar. In 
front of the vascular collar in Scorpio the trunk divides into 
a median and two lateral stems, and from these, arteries are 
given to the cephalothoracic appendages, to the brain and to 
the eyes, as shown in the woodcut. Its main continuation, 
however, is in the vascular collar, the arches of which form 
a large vessel which, as the supra-spinul artery, takes a 
course backwards along the upper surface of the ventral 
nerve-cord (see woodcut, sp.). The association of this part 
of the arterial system with the nerve-cord and its branches is 
very intimate, so as. to have excited special remark on the 
part of Newport. 
A parallel but more intimate association of the correspond- 
