SEA-FISHERIES LABORATORY. 865 
of muscle fibres (fig. 45, Br. m.') underlying the epithe- 
lium but separated from it by connective tissue; these 
run, like the gill filaments, at right angles to the axis. 
More remote from the surface there is a somewhat 
scattered layer of fibres running in the direction of the 
gill axis. Internal to these again. and separated 
from them by more connective tissue is another layer of 
fibres running in the same direction as the first described 
(ite, 45, Bro m."). 
In addition to the above there are two very important 
compact bundles (fig. 45, Br. m.) which run longitudinally 
along the gill axis. They are situated at the sides of the 
axis just above the point at which the various filaments 
separate from one another (Mr. m.). In certain sections 
of P. opercularis these muscles have shown a very similar 
striation to that of the pallial muscles. These last 
muscles serve for contraction of the gills. 
The Cardiac Muscles.—TVhe auricles and still more 
the ventricle are well supplied with muscle fibres. They 
extend around the heart, lying just internal to the wall 
and passing in various directions from the walls across 
the cavity dividing it up, so that it has almost the 
appearance of a sponge. ‘These muscles are described 
with the heart. It is interesting to note here, however, 
that in the specimens fixed and sectioned, no traces of 
definite striation were found on these fibres, except in one 
case, resembling that of the radial, pallial and ctenidial 
muscles, 
FOOT. 
The Foot is a very small organ situated high up on 
the anterior surface of the visceral mass (fig. 1, /.), 
arising from the surface of the gonad close to the mouth. 
In shape, it is roughly cylindrical with a sucker-like 
termination (fig. 7, /’.s.). This free end of the foot 
AA 
