1 66 M. T. Thorell on two European Argulidse. 



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 1^ millim. long. This 

 proceeds with two short strong arms encircling the throat to 

 the hrain, 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 i\\e 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 ai*e somewhat more transparent in the 

 middle part, showing thereby that they originally consist of two 

 lateral halves. 



But it is more especially in the number and course of the nervous 

 stems proceeding from this central chain of ganglia ihatA.coref/oni 

 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 two 

 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. foli- 

 aceus) I cannot venture to determine ; for I entertain doubts on 

 * Ueber Jr^^./o/. p. 329. 



