6.50 MR. GARROD ON THE RESPIRATION OF CHELONIA. [Julie 1 ", 



great difference in the capacity of the thoracic and abdominal cavities 

 which results from differences in the degree of retraction of the limbs. 

 And I also inferred that the activity of the respiratory movements — 

 as in the Lobster, which has some of its larger gills connected with 

 the bases of the ambulacral legs — must depend, in great measure, on 

 the amount of the mechanical force employed in locomotion, in the 

 same way that in the locomotive steam-engine the draught through 

 the boiler-tubes of the furnace depends upon the rapidity of the 

 movement of the engine, because the waste-steam pipe is made to 

 open at the bottom of the funnel. 



A specimen of Trionyx perocellalus (three and a half inches in 

 length of carapace), which had died a day or two previously, lying on 

 my dissecting-room table with its neck and limbs fully extended, I 

 happened to take it up by the lateral margins of its shell, when, upon 

 grasping it between my fingers and thumb, I noticed, to my surprise, 

 that its head and limbs immediately retracted to their full extent. At 

 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 

 completely withdrawn from view and the head fully retracted — the 

 cervical 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 exteuding the 

 head and limbs and closing the orifice, full retraction followed lateral 

 compression, 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 conditian 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 within 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 to a single large Egyptian Trionyx, which is unmanageable 

 and of a more rigid build than the one above described 



