Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.2. Digestive System
The foregut of crabs, like that of lobsters, has in its walls many ossicles, or small hard plates and projections, that ar¬ticulate with one another in a complicated way and serve as a place of attachment for muscles that move the foregut. Ac¬cording to American biologists Robert Pyle and Eugene Cro¬nin, the blue crab has in or associated with its foregut at least 50 ossicles and over 80 muscles. These effect a churning action of the foregut and a grinding of the gastric mill that break down particles of food that have been swallowed. The gastric mill of crabs resembles that of lobsters in consisting of one dorsal and two lateral teeth situated at the constriction that separates the large anterior chamber of the foregut from the smaller posterior chamber.
The midgut originates approximately where ducts from the midgut glands enter the posterior chamber. Behind this chamber, the midgut appears as a small tube, scarcely three- eighths of an inch in length in a full-grown blue crab.
The midgut glands consist of three pairs of lobes, one pair extending forward and to the sides, a second pair extending laterally toward or over the gills, and a third pair leading back toward and, in some species, into the abdomen. The midgut glands may fill much of the body cavity, although their extent at any one time depends largely upon their content of food reserves and water.
As in shrimps and lobsters, digestion of food in crabs takes place partly in the anterior chamber of the foregut, partly in the posterior chamber, and partly within tubules of the mid- gut glands. A bristly filter in the ventral wall of the posterior chamber prevents all but the most finely divided material' from passing up the ducts into tubules of the midgut glands. Since the lumen of the midgut glands is continuous with that of the midgut, these glands are diverticula of the midgut.
In the blue crab, another diverticulum arises from the mid- gut just behind the posterior chamber. A pair of tubes, known as midgut ceca, runs from the dorsolateral surface of the midgut forward and laterally, ending in coils that lie just above the first large lobe of the midgut glands. These ceca are translucent and difficult to see in dissection. Lining the lumen of the midgut ceca are cells like those lining the midgut. Both groups of cells probably function in the absorption of food.
The hindgut makes up the remainder of the digestive tract. It runs between the lobes of the mid-gut glands, under the heart, and into the abdomen, where it follows a straight course to its posterior opening, the anus. Only a slight swel¬ling is present in the most posterior portion of the hindgut, hardly enough to-justify calling, this region a rectum. The entire hindgut is lined with chitin.
In the second or third abdominal segment, the hindgut of the blue crab gives rise to a cecum. From its origin on the left side, the cecum runs forward and over the hindgut, terminat¬ing in closely packed coils on the right side. The function of this cecum is not clear, but this organ may be involved in the regulation of salts in the hemolymph when a crab is exposed to dilute media.