SIR JOHN LUBBOCK ON THYSANURA. 291 
Ventral Tube. (Plate XLV. figs. 10, 11, 12.)—I have already described the arrange- 
ment of the muscles belonging to this curious organ in my previous paper; but I refer 
to it again because I think the figures now given will make the mechanism more clear. 
As I have there pointed out, it is evident that the protrusion of the two filaments 
could not be effected by muscular influence, excepting, indeed, by the indirect effect of 
those muscles which contract the cavity of the body, and thus, intensifying the general 
pressure, squeeze out as it were the two filaments. 
For retraction, however, there are two muscles, a’ b and č d. The first is attached to 
the extreme end of the filament (which in Plate XLV. fig. 12, is represented as par- 
tially retracted); it passes all along the filament, and then close to the walls of the body, 
between them and four lateral muscles, which tend to keep it in place, and is then 
attached to the back, not far from m, and near the median line. The other muscle 
(c' d^) is attached to the middle of the filament; it runs parallel to the preceding, also 
passes between the four lateral muscles and the skin, and divides into two ends, which 
terminate near one another, and still close to the central line of the body. One of the 
filaments, in a state of almost complete extension, is represented in Plate XLV. fig. 11. 
In Plate XLV. fig. 10 are seen the same parts during retraction: e'is the filament, 
turned, of course, inside out; and the muscles are lettered as before. 
The presence of two muscles instead of one is necessary, owing to the length of the 
organ. It is evident that by itself the muscle a b would be able only partially to with- 
draw the filament, the length of the latter being greater than the distance from the 
insertion of the muscle to the end of the ventral tube, e”; the terminal portion of the 
. filament would therefore still be left outside, if it were not for the muscle c' d. More- 
over, when the organ is about to be protruded, it is probable that the muscle c' d relaxes 
first, and thus the filament passes out regularly, whilst, if there were only one muscle, 
it might from its length, get into a knot. 
Muscular System of Tomocerus. 
I will now pass to the abdominal muscles of Tomocerus, and will more particularly 
describe those of the 3rd and 4th abdominal segments, as these are specially concerned 
in the mechanism of the spring. 
Fourth Abdominal Segment. (Plate XLVI. figs. 18, 19, 20.) —If we commence the 
description of the abdominal muscular system from the dorsal line, the first muscle we 
shall meet with is a straight wide muscle (1), which lies on each side of the dorsal vessel, 
rises at the front margin of the segment, and passing directly backwards, through the 
segment and over the intersegmental membrane, is attached to the anterior margin of 
the following segment. In several cases I have found it continuous with the corres- 
ndin scle of the preceding segment. . 
e rt lies en Seiser and therefore, in Plate XLV. fig. 7, is 
under and hidden by it. In some specimens it is straight, in others, as in the one 
figured, it lies a little diagonally. 
- 8 crosses under, or rather outside, no. 2. 
anterior margin of the segment, and passes to that of the 
Like the two preceding, it rises at the 
penultimate one. 
