SIR JOHN LUBBOCK ON THYSANURA. 293 
bouring portions of nos. 6 and 7, as well as of the muscle no. 16, which has not yet been 
described. The lower branch terminates in the third segment before arriving at its 
anterior margin; but the upper branch is longer, and penetrates into the next segment. 
9 rises below, and close to, no. 8. It has the same general course, but diverges 
somewhat, and ends at about the middle of the segment. It will be seen, from my 
drawing, that the end lies under the muscle marked no. 16, and over that marked no. 15. 
It cannot, therefore, be directly attached to the skin in this position, because the muscle 
no. 15 comes directly in the way. The true attachment of this end of the muscle, like 
that of nos. 6 and 7, and the posterior branch of no. 8, with all of which it is closely con- 
nected, is not easy to make out. I believe, however, that a delicate membrane passes 
from its lower extremity, both backwards and forwards, over no. 15, and that, by thus 
acting on the centre of that membrane, the same effect is produced as if the muscle were 
actually attached to the skin at the point where it terminates. 
10 rises under the posterior terminations of nos. 5 and 8, and passes forwards with a 
slight inclination downwards. It passes on the outer side of nos. 6 and 7, and is attached, 
in the ordinary way, to the anterior margin of the segment. 
11 lies outside the preceding. Some of the upper fibres did not, in the specimens I 
examined, reach so far as the margin of the segment. 
19. No. 12, which belongs partly to this segment and partly to the antepenultimate, 
has already been described. | 
13. This powerful muscle rises by a double head. One portion lies under no. 12, 
which is eut away at that part in Plate XLVI. fig. 19, in order to show no. 13; the 
other and larger portion is attached by a broad base to the gently eurved arch, which 
` here runs along the segment. Both branches soon unite, and pass straight forward 
completely through this segment, to the anterior margin of the second. 
14. This and the following are short, but powerful, transverse muscles. The present 
one rises under no. 13, but extends somewhat in front of it, and passes downwards and a 
little forwards to the ventral part of the segment. 
15. This muscle is attached, like the preceding, which it crosses, to the gently curved 
arch already mentioned. It is attached above, in front, and below behind the preceding 
muscle, outside of which it runs. These two muscles are fan-shaped ; and though their 
ventral ends are of moderate size, their dorsal extremities are so wide that between them 
they extend over the whole length of the segment. 
16 also rises by two heads, and lies outside no. 
the thorax, and is attached close behind the head. 
Second Abdominal Segment.—In this segment the dorsal museles are much weaker 
than inthe preceding. In several cases I even found nos. 1, 2, 3, and 4 entirely deficient. 
No. 5, on the contrary, seems to be always present. There are, indeed, two muscles 
marked thus. The one rises in the third abdominal segment, as I have already men- 
ards, dividing into two heads, which are attached to the anterior 
margin of this segment. The other, which corresponds with it, rises at the posterior end 
of this segment, at the edge of the muscle just described, and, passing through the 
present segment, is attached to the anterior margin of the first abdominal segment. 
15; it passes forward, straight into 
tioned, and passes forw 
