11 



cavily, the iiinor side ot wliicli, c'xcc'[tt in I'loiil, is IoiiiumI 

 by the side of the foot. Above the liead a \-dv^v iransveise 

 opening- knids into tlie nuchal eavity already mentioned. 

 This may be regarded as a local deepening of the pallial 

 cavity, originally formed as a " branchial cavity " to 

 shelter Ctenidia. 



Projecting from the inner surface of the mantle are a 

 large number of plate-like pallial gills, Avhich are flattened 

 in a direction perpendicular to the curve of the mantle 

 outline. They form a continuous series right round tlie 

 bod}', except at the left end of the nuchal openiiig (fig. -J) ; 

 at this point the pallial vein, whicli conipletely encircles 

 the gills, runs in to enter the heart. These pallial gills 

 are secondary structures, which have replaced the primary 

 gills, or ctenidia, just mentioned. 



After removal of the shell tJie following points can be 

 made out in dorsal view (fig. -1) : — 



(a) The shell muscle shaped like a horse-shoe with 

 forwardly directed thickened ends. It is divided into a 

 variable number (1^-lT) of fasciculi, separated by 

 transverse fibrous septa. 



(h) The mantle skirt running right round the body 

 external to the shell muscle. In front it extends back- 

 wards for a short distance between the ends of this 

 muscle to constitute the roof of the nuchal cavity. Just 

 external to the shell muscle is seen the band of attach- 

 ment of the pallial muscle to the shell. Just internal to 

 the anterior part of this attachment band there is a 

 very well marked second line of attachment running 

 between the tips of the shell muscle and indicating the front 

 boundary of the nuchal cavity. This is the internal pallial 

 muscle. The mantle is pigmented dorsally near its edge, and 

 so also is its band of muscular attachment, but the attach- 

 ment zone of the internal pallial muscle is not pigmented. 



