Digestion in Molluscs and Arthropods

A.  Digestion, Absorption and Assimilation in Molluscs
     1.  Bivalves--
          a.  Food is collected on lamellar gills and 
               carried to mouth area.  Creature uses its
               palps to sort the items that are contained
               in a muscous string.
          b.  Muscous string is wrapped around a 
               crystalline style that protrudes from
               from the style sac.  The crystalline style
               is actually composed of digestive enzymes
               in a matrix.
          c.  As the materials wrap around the style, the
               carbohydrate materials in the mucous string
               are broken down by carbohydrases.
          d.  Food is then carried into stomach and then
               into digestive gland.  Materials are then
               digested intracellularly by the cells lining
               the digestive gland.  For many of the 
               bivalves, the storage product is glycogen
               and this is what gives the clam a somewhat 
               sweet taste when eaten
          e.  From the stomach/digestive gland junction a 
               small intestine proceeds to the vicinity of
               the excurrent siphon.  One curiosity is that
               the intestine passes through the middle of
               the venticular chamber of the clams heart 
               (actually the heart is wrapped around the
               intestine).
     2.  Squid--
          a.  The squid have a long esophagus that carries
               material to the stomach. The stomach is
               designed for the rapid breakdown of protein
               material.  Adjacent to the stomach is a
               caecum which stores the ingest chunks of 
               food until they can be passed on to the
               stomach.
          b.  Process is very quick, as retention of the 
               undigested materials would impede the 
               squid's locomotory abilities. Times for 
               digestion are in terms of a couple of hours
               rather than four to six hours of other 
               organisms.
          c.  Waste materials are carried by the small
               intestine to the anus which is typically 
               located in the mantle cavity in the 
               vicinity of the siphon.
     3.  Grazing snails
          a.  After material is scraped off the substrate
               it is passed into the digestive tract going
               first down the esophagus into the stomach and
               then into the intestine.  The intestine is
               very long.  This not only facilitates the
               breakdown of cellulose, but also the up-take
               of the materials.
          b.  Because cellulose is rather indigestible, this
               process requires a long processing time.
B.  Arthropods
     1.  Digestion, absorption and assimilation in crayfish
          a.  Crayfish has series of mouthparts, eg., 3 prs
               maxillipeds, 2 pr maxillae, and one pair mandibles
          b.  Maxillipeds and maxillae sort and strain food items.
          c.  Food items pass into two-compartmented stomach.
               First part of stomach has chitinous lining.  Food 
               is triturated in this part of the stomach, i.e., 
               food is mixed with enzymes and broken up into 
               small components.
          d.  There is a valve-like structure between the two 
               sections of the stomach and only when particles
               are small enough can they pass into the second
               part of the stomach.  This valve is composed of
               setae that pass towards the middle of the 
               opening and act collectively as a sieve.
          e.  Crayfish have a digestive gland (often called the
               tamale in restaurants).  Food materials complete
               the digestive process by intracellular means.
               As in the clam, the storage product is often
               glycogen or a glycogen related material.  Because
               of this the tamale is also "sweet" tasting.