32 
STIGMATIC OPEECTILA IN THE SCOEPION. 
is visible^ the other half being overlapped by the portion 
of the sternum which is in front of the stio-ma. 
The free edge of the operculum is continued into a thin 
membranej whichj together with a similar one attached to 
the anterior edge of the stigma, and to the inner face of the 
part of the chitinons sclerite in front, is joined to the axis 
of the lung-book. Air passes between these two to the 
lung-book. 
Attached to the stigmatic operculum all along its free- 
edge, are a series of small muscles which arise from the 
inner face of the sternum a little behind the posterior edge 
of tlie stigma, and have hitherto been undescribed ; I pro- 
pose to call these the "opercular muscles" No muscles 
attached to the operculum itself, which pull it in the opposite 
dii'ection, are present, so far as I can see. 
How does this apparatus work ? When the opercular 
muscles contract, the operculum is drawn back and the 
stigma opened. When they relax, the elasticity of the 
operculum is brought into play, and it is shut. It is possible 
that muscles attached to the lung-books or to the thin 
membrane which is continuous with the f i-ee-edge of tlie 
operculum, may help in closing the stigma. The pressure 
of till above-lying- tissues and organs may also have some 
effect in this direction. 
The curiously modified chitiu of the anterior part of the 
operculum has an important function. When it is applied 
asrainst the membrane attached to the anterior edo^e of the 
stigma in the act of closing the last, the membrane 
and the soft tissue which lies below it will be thrown into 
innumerable little projections, whicli will fit iu into the 
spaces between the chitinons rods. Thus not only is the 
operculum held on lirnily lo the anterior edge, but the 
access of water and other Ihiids to the pulmonary cavity is 
rendered impossible. 
