166 M.T. Thorell on two European Argulide. 
Internal structure.—As is the case in A. foliaceus, the cen- 
tral portion of the nervous system is highly concentrated. Es- 
sentially the two species agree in what concerns this portion of 
the nervous system; but there are still many differences which 
it is not difficult to detect. In A. coregoni the ventral series 
of ganglia reaches only from the throat to the base of the 
second pair of footjaws, and is about 15 millim. long. This 
proceeds with two short strong arms encircling the throat to 
the brain, which seen from above is almost quadrate, though 
somewhat longer than broad, rounded behind, diminishing 
somewhat anteriorly, and here giving off the two strong nerves 
for the organs of vision. From above the brain shows the 
usual portion dilated into three lobes and bears the single 
ocellus. The ventral cord consists of six ganglia, which are 
situated close upon one another, without any well-marked 
commissure, but are all evidently separate. The first ganglion 
is much longer than the others, rounded, and separated by 
a stronger constriction from the rest of the series. The five fol- 
lowing ganglia form a common, oblong division, which is some- 
what diminished towards the extremity. The last ganglion is 
rather longer and smaller than the foregoing, the breadth of 
which is more than double their length. They are bent inwards 
on the hinder surface, and are somewhat more transparent in the 
middle part, showing thereby that they originally consist of two 
lateral halves. 
But it is more especiallyin the number and course of the nervous 
stems proceeding from this central chain of ganglia that A. coregont 
differs from A. foliaceus, at least according to the exposition of 
the nervous system of this latter species given by Leydig *. In 
A. foliaceus the ventral cord is similarly composed of six gan- 
glia, but, strangely enough, no nerve-stems proceed from the 
second, fourth, and fifth of these. The first and third each give 
off one pair of nerves, the sixth three pairs, of which the inner- 
most (last) passes into the head-shield, the others branching off 
to the second pair of footjaws and the legs. With A. coregoni, 
however, the case is quite different. The first ganglion gives off 
on either side one strong stem (as in A. foliaceus, where this 
stem gives off one branch to the sucking-cups, another to the 
second pair of footjaws) ; from each of all the following ganglia 
proceed two pairs of nerves; and these nerves unite with ¢wo 
bundles extending, one on each side of the trunk, as far as the 
tail. The further course of the various nervous stems I have 
not been fortunate enough to make out clearly: whether some 
of them branch forward or become fused together (as in A. folt- 
aceus) I cannot venture to determine; for I entertain doubts on 
* Ueber Arg. fol. p. 329. 
