A crop-like dilatation of the gut and a recurved intestine, embedded in the compact yellowish-brown liver, the ducts of which open into it, form the rest of the digestive tract and occupy a large bulk of the visceral hump. The buccal region presents a pair of shelly jaws placed laterally upon the lips, and a wide range of variation in the form of the denticles of the lingual ribbon or radula.
Vinen, loc. cit.), with gallic and ellagic acids, ligneous fibre, water, and minute quantities of proteids, chlorophyll, resin, free sugar and, in the cells around the inner shelly chamber, calcium oxalate.
A further variation consists in the development of additional shelly plates upon the dorsal line between the two large valves (Pholadidae).
But the shelly substance does not always in boring forms adhere to this form after its first growth.
In Aspergillum the whole of the tubular mantle area secretes a continuous shelly tube, although in the young condition two valves were present.