520 ON THE RESPIRATION OF CHELONIA. 
first I was inclined to attribute the movement to the persistence of 
muscular irritability in the recently dead individual, but, on making a 
cut into one of the limbs, found that this was not the case. 
As frequently as I chose to extend the head and limbs to their full 
extent they so remained until the body was laterally compressed, 
whether it happened to be lying on its abdomen or on its back, or in 
any other position. Immediately it was pinched the limbs were com- 
pletely withdrawn from view and the head fully retracted—the cervi- 
cal region of the spine, from being straight, assuming the curve 
essential to the cephalic retraction. 
To determine the mechanism of this unexpected movement was 
my next proceeding; and I made a small hole in the centre of the 
plastron which opened into the body-cavity. I then again, with the 
limbs and head extended, repeated the lateral compression, and found 
that they were no longer retracted as they had been previously, air 
rushing in at the newly-formed opening. Upon extending the head 
and limbs and closing the orifice, full retraction followed lateral com- 
pression, as at first. This experiment was repeated several times 
with the same result. 
It then became evident that in laterally compressing the plastron 
(which in the extended condition projects beyond the margins of the 
carapace) its slight convexity is increased, and that this is associated 
with an augmentation of the capacity of the body-cavity, which, to fill 
the deficiency thus produced, causes an insucking of the head and 
limbs upon simple pneumatic principles. The retraction of the head 
and limbs is therefore nothing more than a movement of suction, and 
does not depend upon any important direct osteological or myological 
peculiarities of the animal. 
Whether the creature is in the habit during life of employing this 
suction method of withdrawing itself with its shell is a question that 
I am not able to determine just now, as the number of Soft Tortoises 
living at the present time in the Society’s Gardens is reduced toa 
single large Egyptian Trionyx, which is unmanageable and of a more 
rigid build than the ene above described. 
