STIGMATIC OPEECULA IN THE SCORPION. 
33 
What is the disposition of the stigmata in the living- 
animal under ordinary conditions ? Sometimes they are shut^ 
though generally one or two are slightly opened ; some- 
times some are shut and some opeu^ more often partially 
than completely. The opercula often remain for a long time 
without moving ; this accounts for the fact that both Dufour 
and Plateau failed to observe any movements connected 
with the stig-mata. But these movements do occur. Accor- 
ding to Plateau, respiratory movements such as are found 
in Hexapod-insects are absent in scorpions. But -whether 
this is the case or not, it is desirable for the scorpion to be 
able to regulate the supply of air to its wants at differeut 
times. I think it is very probable that these movements 
serve that purpose. 
But though no movements connected with the stigmata-, 
i.e. of the operculum, may be observed even for a long time, 
it is easy to demonstrate their existence by touching the 
stigma Avith the blunt end of a needle or b}^ ^Dutting a drop 
of water on it when it is open. It is at once closed by the 
operculum. On the other hand, if the stigmata are shut, 
it is only necessary to hold the scorpion for a minute or two 
under water ; when taken out the stigmata will be found 
to be wade open. This latter circumstance is a little re- 
markable, seeing that in the scorpion as in most other in- 
vertebrates,, metabolism is very slow. The scorpion will 
live for hours under water or in nitrogen gas; and ordinari- 
ly its stigmata are shut for a long time. Perhaps, it only 
indulges in a sense of freedom from recent restraint. 
I tried several experiments with a view to ascertain 
whether the stigmata would be shut when the animal is 
exposed to irritating vapours and gases. When placed in 
ammonia vapour or carbonic acid gas, the stigmata if 
previously open, do not shut. Dr. Bourne has sug- 
gested to me that substances which are irritant in the case 
of man, may not be so in lower animals. It is possible 
