Mitchell,, on a Pulsatile Muscular Organ. 37 
tion of the sac. Its function therefore being that of an 
additional impelling organ^ it may be termed an auxiliary 
heart. 
This organ, there is good reason to believe, consists of two 
chambers, for by altering the focus of the microscope a double 
current can be observed on each side' of it. These currents 
are distinctly seen to pass close up to it, but are then lost 
sight of, and their passage through it cannot be observed. 
This may be caused by the energy and rapidity^ of the pulsa- 
tion, and also by the organ at each contraction being thrown 
into transverse rugae, which render it more opaque. At the 
exact instant of its coming to rest, however, a few discs may 
be seen within it, but these are speedily lost sight of, as is 
also the organ itself, for being of the same colour as the rest 
of the tissues of the limb, its existence can only be plainly 
seen while it is puckered when in action. A darkish curved 
line is all the indication of its existence when at rest. 
The organ does not occupy the same position in all the 
legs. In the anterior pair it is in the broadest part of the 
claw-like tarsi close to their junction with the tibise ; it was 
here I first detected it. I afterwards found the same organ 
in the tibia, close to the knee-joint, in the second and third 
pairs of legs. 
The organ in question is found in the very young and 
small larvae as w^ell as in the pupa and perfect insect. In 
some pupse, kindly furnished to me by the Honble. Walter 
Elliott, upw^ards of two inches long, the pulsating organ was 
so large that it could be well seen with a two-inch objective ; 
and, with the one-inch, at every contraction it was seen 
to be throw^n into the transverse folds previously men- 
tioned, the edges of the folds having the appearance of 
little cords pulling at the concave margin of the sac, which is 
always the most visible part of the organ. The heart appears 
to expand again by its own elasticity ; for though one or two 
cords are seen to extend in a longitudinal direction, they 
probably do little more than keep the organ in its place. 
In some Notonectidse, also, after a little trouble I discovered 
a similar movement, but it was so much more difficult to see 
that it would in all probability have escaped detection had I 
not been specially in search of it. 
In a small transparent water-beetle I could not find it; 
neither could I detect anything like it in the larva of Agrion 
or of Ephemera. 
In the foregoing I have endeavoured to record what the 
* 200 per minute, measured with a metronome. 
