542 
J. S. KINGSLEY. 
accords well with the facts just noted. The anterior appendages 
•were thus brought further from their nerve-centres and hence 
(if I may use the expression) they exert a tractive force first 
on the nerves which supply them and secondarily on the ganglia, 
for it is for the evident advantage of the animal to have the 
nerve-centres nearer the parts most used. 
The foregoing account mentions five postoral and pregenital 
ganglia ; the remaining one, the first of the series, has, as far 
as I can see, been merged in the cesophageal collar, and the 
corresponding pair of nerves (which supply the small first 
pair) appear to arise from the outer surface of the collar at 
about a level with the posterior margin of the brain. Of the 
exact position, however, I am not quite certain. 
I have spoken above of the commissures, and while not sure 
of their origin, I am confident that the connectives do not 
arise as secondary epiblastic invaginations, but merely as dif- 
ferentiations of the primary neural ridges. With the speciali- 
zation of the ganglia, which is brought about by a more rapid 
increase in size, the formation of the fibrous portion of the 
nervous system begins. In transverse section this has a 
granular appearance with scattered superficial nuclei, which 
may play a part in the formation of the neurilemma. The 
nerves to the appendages arise as outgrowths from the ganglia 
which extend themselves among the mesoblastic tissues (fig. 
35). These outgrowths contain but few nuclei at any stage 
but are mostly fibrous, and are directly connected with the 
corresponding portion of the central nervous system. 
One of the most marked peculiarities of the adult Limulus is 
the fact that the ventral nervous cord is ensheathed by the 
large sternal artery, a fact without parallel except in the 
Arachnids. The early stage of this artery is shown in fig. 36. 
At all times there is a considerable space between the meso- 
blast and the nervous system, and at this time processes begin 
to grow out above and below, on either side, from the meso- 
blast. These in the section figured partially embrace the 
cords at a later stage the two halves unite and form the walls 
of the artery. 
