SEA-FISTIERIES LABORATORY. 355 
in the act of swimming and the necessity of withdrawal 
and protrusion of the edge with its numerous sensory 
structures. It has both radial (fig. 3, P. MW. r.) and what 
may be termed concentric muscles; the latter extend 
round the margin of the mantle parallel to its free edge, 
and are well developed in the velum (fig. 4, V. M.c.), 
which has a very compact muscular structure. 
The radial muscles are the most obvious when 
examining the mantle, for it is these which attach the 
mantle edges to the shell and retract them when the 
valves close. 
The line of attachment on the shell has been 
previously seen to be a continuous line extending almost 
parallel to the shell margin and at some distance from it, 
furthest at the ventral edge and approaching it anteriorly 
and posteriorly. These pallial muscles proper arise, 
where attached to the shell, as slightly separated bundles 
of fibres, as if, in fact, a bundle had the end frayed out 
slightly. These separated fibres almost immediately 
come together again to form a conspicuous large fibre 
which radiates out to the margin and breaks up into very 
numerous finer bundles, which interlace and become 
crowded together as they reach their termination at the 
base of the velum. 
Between the outer pallial fold bearing the tentacles 
and the median one bearing tentacles and the eyes, there 
is a deep groove, known as the Periostracal groove 
(figs. 4, 6, P. gr.), and in sections the periostracum can be 
seen arising from the base of the groove through the 
coalescence of several short fibres from the secreting cells. 
From here it is continued out, and passes over the edge 
of the shell to its outer surface. 
At the bottom of the groove lying along each side 
there is a ridge formed by much elongated epidermal cells, 
