400 Dr. F. Miiller on Balanus armatus. 
gin of the tergum. The rows of bristles upon the lines of 
growth of the opercular pieces are more strongly developed in 
Balanus armatus, especially upon the scutum, Ke in the few 
other species which I have been able to compare with it. 
They are short and delicate upon the carinal side, and 0°2 
millim. or more in length, and delicate, upon the scutal side of 
the tergum, of the same length, but far thicker and closely 
packed, upon the scutum. Longer and shorter bristles alter- 
nate, but not as two clearly different forms, as is the case in 
B. improvisus, var. assimilis. The chitinous cords (tubuli, 
Darw.) which, proceeding from the bristles, permeate the oper- 
cular“pieces in an undulating course, become rapidly dimi- 
nished into delicate filaments, which may easily be extracted 
from the surrounding mass by breaking up the opercular pieces 
when deprived of lime by acids (figs. 14-16) *. 
points in the neighbourhood of the spurs. Here the operculum is closely 
applied to the sheath; the membrane which unites the base of the oper- 
culum with the sheath is here narrower and firmer than elsewhere. Thus 
these two points form an axis (which is certainly somewhat displaceable) 
round which the operculum can turn. We may now, in fresh animals, 
readily seize the individual muscles and pull them, so as to arrive at an 
explanation of their action. As might be expected from the mode of 
attachment of the operculum, the carinal margin of the tergum is drawn 
down by the depressores tergi, whilst, on the contrary, the rostral angle of 
the scutum is elevated and the occludent margin acquires an almost hori- 
zontal position, These muscles alone effect the powerful holding-down 
of the closed operculum; the. latter is then supported by the carinal mar- 
gins of the terga against the sheath, which at this point usually shows 
more or less distinct traces of wearing. By the depressores scuti, both 
laterales and rostrales, the bases of the scuta are drawn down, and the ca- 
rinal margins of the terga elevated, whilst the occludent margin attains a 
more or less upright position. Because its apex projects further, the 
entire operculum then appears elevated ; this, however, is only apparent, 
and this elevation of the apex may be produced by pressing from without 
upon the rostral angle of the scuta, instead of pulling from within upon 
the depressores scuti. Elevation and sinking of the entire operculum, 
however, takes place, only to a rather limited extent; how far a pressure 
of the body against the base assists in it, I will leave undecided. 
The opening of the operculum is brought about, I believe, only by the 
pressure of the animal against the opercular fissure ; the depressores late- 
rales cannot open it. As may be easily ascertained from opercula which 
have been removed with their connecting membrane, the two halves of 
the operculum, in opening and closing, turn upon an axis passing through 
the rostral angle of the scuta and the carinal angle of the terga; what- 
ever is situated above this axis departs during the opening from the me- 
dian line, and whatever lies beneath it approaches this line. But the 
depressores scuti laterales pass from points beneath the turning axis 
downwards and somewhat outwards, and therefore cannot possibly ap- 
proximate their points of insertion to the median line, as would be neces- 
sary for the opening of the operculum. : 
* In Tetrachta porosa, in which the chitinous cords remain tolerably 
thick to the end, I have seen a pale filament projecting from their extre- 
