23 



ill the act of swimmiiig and the necessity of withdrawal 

 and protrusion of the edge with its uumerous sensory 

 structures. It has both radial (fig. 3, P. M. 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. 



