446 MR. LUBBOCK ON THE THYSANURA. 
as may be seen in fig. 36, very nearly divides the tentacle into two equal parts, though 
the basal portion (that is to say, the part /, b, a) is slightly the longer of the two ; 
although, therefore, the part i, c,f is always found bent nearly into a semicircle, still the 
tentacular ends of the two muscles e and h are kept about ^th of an inch apart, while their 
opposite ends are attached, close together, to the wall of the back, immediately above the 
ventral tube — a position which adds much to the difficulty of dissecting out the whole 
organ without injuring any of the parts. The terminal muscle h is single and cylindrical, 
while the other one is divided nearly in the middle into two equal branches (fig. 36, e, g). 
Although the arrangement and attachments of these different parts are so peculiar and 
intricate, a comparison of the two figures (PI. XL VI. figs. 35, 36) will, I think, satisfactorily 
illustrate the mode in which the organ is extended, directed, and retracted. The whole 
skin of the animal is kept in a state of considerable tension by the quantity of its contents ; 
so that if an incision is made, part of the entrails are immediately forced out. If, therefore, 
we imagine the muscle e to relax, the tension of the skin acting on the place at which 
t he ventral tube is folded inwards — in fact, on the tube b, at the end a — will begin to force 
the tentacle inside out, so that what was the inner wall will become the outer one, and 
vice versa, and the muscle e, as it elongates, from being external to the tube, will gradually 
become internal. The first effect of this upon the other section of the tentacle, c, will 
be to straighten it, but gradually the muscle h also must relax ; the continuance of the 
same pressure will invert also the tube c, and, the muscle h being carried into it in the 
same manner as has already happened to e, we shall at last have the state represented in 
fig. 35, — the whole tube being inverted from its former position, the lower half contain- 
ing the two muscles, the upper half only the one marked h. 
The protrusion of the organ, however, though probably much assisted by the tension of 
the skin, is not to be altogether attributed to it, as the muscles are apparently surrounded 
by a layer of transverse fibres, which must tend to produce the same effect. I was at first 
inclined to wonder at the presence of two muscles, and to suppose that the object might 
have been equally well attained by the presence of only the one marked h. The two 
muscles, however, acting on different parts, may enable the organ to be retracted more 
quickly than might otherwise be the case — an advantage evidently of much importance 
to a part so delicate. Whatever also may be the function of the tentacles, a muscle 
tttached to the middle part will enable the animal to apply this portion to some parts ol 
its body which perhaps could not otherwise be so conveniently reached. Again, it « 
evident that a single muscle attached to the part d could not have fully retracted the 
tube, because the distance between d and a is less than the length of the tentacle; 
but this difficulty is at once avoided by the presence of a second muscle, which throws 
the tentacle into a curve; moreover, greatly as the two muscles are extended when 
the organ is protruded to its full length, if there was only one it would require a still 
greater power of elongation. 
One function at least of the ventral tube is easily visible. If a Smynthurns is laid on 
its back— a position from which it has some difficulty in recovering its feet,— and if, while 
it is in this attitude, a piece of glass is brought within its reach, the animal will endeavour 
to seize it with the feet, but at the same time it will project one or both of the ventral 
