3o6 MOLLUSCS. 



floor. Below the anterior closing (adductor) muscle, the large 

 mouth will be found, bordered beneath by two lip-processes 

 (labial palps) on each side; above the posterior closing 

 muscle the food-canal ends. The whole space between the 

 two mantle flaps is called the mantle cavity, and it is divided 

 by a slight partition at the bases of the gills into a large 

 ventral infra-branchial chamber, and a small dorsal supra- 

 branchial chamber which ends at the exhalent orifice. 



On the valve of the shell folded back, are seen a number 

 of marks — the successive attachments of the mantle margin, 

 the insertions of a number of muscles, and perhaps a few 

 small pearls formed by the enclosure of some minute 

 grains of sand in the prismatic layer. The following muscles 

 are inserted on the shell and leave impressions — (a) the 

 anterior adductor, (fi) the posterior adductor, {c) the anterior 

 retractor of the foot continuous with (a), {d) the protractor 

 of the foot a little below (a), («) the posterior retractor of 

 the foot continuous with (V). As the shell grows the insertion 

 of the muscles and the attachment of the mantle change, 

 and the traces of this shifting are visible. 



Skin. — There is much ciliated epithelium about Anodonta, 

 on the internal surface of the mantle, on the gills, and on 

 the labial palps ; and little pieces cut from an animal incom- 

 pletely dead {e.g., from the oyster which many of us swallow 

 half alive) have by means of their cilia a slight power of 

 motion. A few investigators maintain that the non-ciliated, 

 but glandular, skin of the foot has pores through which 

 water may enter, but histological and physical facts are 

 decidedly against this conclusion. 



Muscular System. — The shell is closed and kept closed 

 by the action of the two adductor muscles. When these 

 are relaxed under nervous control, the elasticity of the hinge 

 ligament opens the valves. A book with an elastic binding 

 stretched when the book is closed by clasps, would in the 

 same way open when unclasped. It is easier for the mussel 

 to operi the mouth of its shell than to keep it shut. The 

 "foot" is a muscular protrusion of the ventral surface, and as we 

 have already mentioned, is under the control of three muscles 

 — a retractor and a protractor anteriorly, and a posterior 

 retractor. Its upper portion contains some coils of gut and 

 the reproductive organs ; its lower region is very muscular. 



