203 



That which first strikes one's attention, in dissecting out tlie 

 nervous system, is the absence of so marked a consohdation of 

 the ventral chain of gangha as characterizes most of that group 

 of insects. Instead of the single oval ganglionic mass, which 

 in the- near relatives of Phylloxera represents the concentrated 

 ventral chain of ganglia found in most arthropods, the Phyl- 

 loxera presents two distinct ganglionic masses below the front 

 end of the digestive canal ; or, to be more exact, two pairs 

 of ganglionic enlargements. The component lateral halves of 

 each of the two masses are so consolidated as to leave only 

 slight external indication of the fusion. This fusion of the 

 lateral halves is, however, like the antero-posterior consolida- 

 tion, not so pronounced as in many of the plant-lice. These 

 two nervous masses are joined to each other by a pair of stout, 

 short, longitudinal commissures. The anterior, in turn, is 

 joined to the superoesophageal pair of nerve-centres by a pair 

 of much longer and more slender commissures. 



The posterior ganglionic pair is larger than either of the two 

 remaining pairs. It is continuous behind with a single median 

 nerve trunk, which promptly bifurcates. As seen from above it 

 presents a somewhat elongated heart-shaped, symmetrical out- 

 line, its greatest breadth being considerably in front of its 

 centre. In a lateral view it is also seen to present the great- 

 est thickness a little in advance of its centre. Its outline from 

 the latter point of view is, however, not quite symmetrical, the 

 dorsal surface being considerably more flattened than the ven- 

 tral. In their backward continuation, moreover, the dorsal and 

 ventral outlines are quite unlike, inasmuch as the former passes 

 with almost unnoticeable curvature into the dorsal outline of 

 the main nerve-trunk, while the ventral outline suffers a con- 

 spicuous and rather prompt bending to bring its course into a 

 line parallel with the dorsal outline of the main nerve. ^ At its 

 anterior end, also, differences in the course of the dorsal and 

 ventral outlines are noticeable. Wiiile the outline of the dorsal, 

 in passing by means of the stout commissure into that of the 

 first suboesophageal ganglion, presents only a very gentle 



^ Owing to the obliqueness of the view furnished by the figure this difference is not 

 there made prominent. 



