Box Extension 6.3

The Digestive Systems of Arthropods and Bivalve Molluscs

The insects and the crustaceans (e.g., crayfish, crabs, and shrimp) nicely illustrate the digestive system of arthropods (Figure A). In them, digestion is principally extracellular. Another similarity to vertebrates is that food is moved through the insect or crustacean digestive tract by muscular contraction.

Morphologists describe the digestive tract of an insect or crustacean as consisting of a foregut, midgut, and hindgut. For defining these, we need first to mention that in these animals the early and late parts of the digestive tract have an ectodermal developmental origin. Because of this they synthesize exoskeleton and are lined with a thin layer of chitinous exoskeleton material, called cuticle. The foregut and hindgut are defined to be the parts of the digestive tract at the anterior and posterior ends that have this cuticular lining (see Figure A). The midgut is of different (endodermal) developmental origin and lacks a lining of cuticle.

Another point deserving early mention is that in insects (but not crustaceans) the urine-producing tubules, called Malpighian tubules, empty into the gut at the junction of the midgut and hindgut (see Figure A1). Thus urine joins other gut material at that point and travels through the hindgut to be excreted by way of the anus. Because the hindgut plays a crucial role in adjusting the final composition and quantity of the urine, it is part of the excretory system as well as the digestive system of an insect.

Figure A The digestive systems of two types of arthropods: insects and crustaceans (1) An insect. (2) A crustacean. Although the excretory system of crustaceans is separate from the digestive system, in insects the Malpighian tubules—which constitute the initial part of the excretory system—connect to the digestive system at the junction of the midgut and hindgut.

Our discussion of arthropods continues in Box Extension 6.3, where you will also find an illustration of the bivalve mollusc digestive system and a discussion of how it works.

Insects (see Figure A1) commonly have an enlarged storage chamber, termed a crop, in the foregut. Where the foregut meets the midgut in some species, the foregut also includes a muscular chamber, called a proventriculus or gizzard, which acts as a grinding organ. The midgut is the principal site of both digestion and absorption in an insect. Numerous digestive enzymes are secreted into the midgut, which is maintained at a near-neutral pH. The midgut typically bears anterior diverticula—called ceca (singular cecum)—that help with absorption and sometimes house microbial symbionts. Some nutrient absorption occurs in the hindgut.

The foregut of crustaceans (see Figure A2) consists of a tubular esophagus followed by a saclike structure called the stomach (proventriculus). Acidic conditions are created in the stomach during digestion, but not nearly as acidic as those in the human stomach; the pH of stomach fluids in crustaceans is 4 or higher, although the pH can be lower than 1 in humans and other mammals. The stomach is often divided into two chambers. The anterior (“cardiac”) chamber is particularly muscular, and often part of it is specialized as a gastric mill. The cuticular lining of the mill is thrown into ridges with cutting edges, and the mill grinds food, frequently with the aid of digestive enzymes that arrive in the anterior stomach chamber by retrograde movement from the midgut. The second stomach chamber of a crustacean—the posterior (“pyloric”) chamber—is smaller than the first. Its cuticular lining often bears bristlelike setae (singular seta), which act collectively as a sieve, keeping food particles from leaving the foregut until the gastric mill has reduced them to a small size.

The midgut of crustaceans varies in complexity from being a fairly simple tube in some species to bearing elaborate arrays of anterior and posterior diverticula (ceca) in others. The hepatopancreas, an organ of enormous importance in crustacean digestion, connects to the anterior midgut via ducts. The hepatopancreas is poorly named because its name greatly exaggerates any similarity it bears to the vertebrate liver and pancreas. The hepatopancreas consists of a branching array of blind-ended tubules lined with epithelium. One of its several functions is to secrete digestive enzymes that flow by way of its ducts into the midgut and then retrograde to the stomach. Food particles from the midgut also enter the hepatopancreas, and much extracellular digestion and absorption occur within the hepatopancreas. It is both the principal source of digestive enzymes in crustaceans and the principal site of nutrient absorption. In addition, the cells of the hepatopancreas play important roles in storing lipids and glycogen.

The bivalve molluscs—clams, mussels, oysters, and their relatives—are ecologically important animals that provide an outstanding example of how very different from vertebrates some animals can be in their digestive and absorptive processes. The gross morphology of the digestive tract in bivalves is not particularly remarkable. Bivalves are said to have an esophagus, stomach, intestine, rectum, and so forth. Striking differences from the vertebrates and arthropods come to light when details are examined, however. First, bivalve molluscs depend strongly on ciliary action, rather than muscular contraction, to move food through their digestive tracts. This is important in part because ciliary action is capable of sorting particles according to size and other features. A second major difference is that in many bivalves, digestion is primarily intracellular.

The key to understanding bivalve digestion is to understand the complex array of processes that take place in the stomach (Figure B). One striking feature of the stomach is the presence of a long, slender gelatinous rod called the crystalline style. Many an oyster lover has been startled to stumble upon this dramatic structure—as perfectly clear and colorless as ice—while preparing fresh oysters for eating. It is synthesized by the style sac. Because the protruding end of the style disintegrates during food processing, as we will see, new style material must be added regularly by the style sac. One end of the style remains sheathed by the style sac, and cilia within the sac turn the style on its long axis.

Figure B Food processing in the stomach of a clam In bivalve molluscs such as clams, the stomach and associated digestive diverticula are the most critical parts of the digestive system. Materials are mostly moved about by ciliary action. Turning of the crystalline style draws a mucus strand containing food particles into the stomach and, by abrasion against the gastric shield, releases digestive enzymes from the style material. In many species, digestion occurs primarily within the epithelial cells of the digestive diverticula, as depicted here. The digestive tract inside the clam is presented diagrammatically, not realistically.

The style has several functions. One is to help pull strands of mucus, containing collected food particles, into the stomach from the mouth. It does this by winding the strands onto itself. The end of the style that protrudes from the style sac pushes against a chitinous plate, the gastric shield, on part of the stomach wall. The turning of the style against the gastric shield helps break up food particles by mechanical action. In addition, the material substance of the style contains digestive enzymes, particularly amylases (starch-digesting enzymes), which are released into the stomach cavity as the turning of the style against the gastric shield causes style material to disintegrate. Once released, the enzymes carry out extracellular digestion.

The wall of a bivalve’s stomach is lined with complex ciliary fields, which sort food particles and direct them along intricate paths. Of particular importance, ciliary action carries materials into and out of digestive diverticula connected to the stomach. These diverticula (often misleadingly called the digestive gland) are composed of branching, blind-ended tubules and represent particularly critical components of the bivalve digestive–absorptive system (as well as being major storage sites for lipids). The digestive diverticula, in fact, are the principal sites of digestion and absorption in many species of bivalves. Food particles are carried into the diverticula by certain ciliary tracts. The food particles are then taken into cells lining the diverticula by phagocytosis and pinocytosis. The cells possess intracellular digestive enzymes, and much of digestion occurs intracellularly. The nutrient products of digestion are believed then to be passed into the blood bathing the diverticula.

In some types of bivalves, the tubular midgut is also implicated in digestion and absorption. A bizarre and poorly understood aspect of the bivalve digestive tract is that the hindgut passes directly through the ventricle of the heart on its way to the anus!