Physiological Processes (Continued)

The digestive, excretory, respiratory, circulatory, and reproductive systems lie within the cephalothorax, under the protective carapace. These systems are quite similar for individual species within the class Decapoda. However, the nervous system, running through both the cephalothorax and the abdomen, does differ, with that for clawed lobsters being more primitive than that of the spiny lobsters.

Digestive System

The digestive system consists of a long tract divided into three main regions: the foregut, midgut, and hindgut. Food is handled and processed first by the mouthparts and then is crushed by the mandibles before being swallowed. Afterwards it enters the muscular esophagus which is lined with mechano- (touch) and chemoreceptors (taste) and is capable of great distention. When the sensory organs within the esophagus are stimulated, they cause a rhythmic contraction and relaxation of the muscles lining the esophagus which helps to push food into the anterior chamber of the stomach, or the cardiac stomach. This chamber contains folds that permit it to expand and fill with food. It usually has numerous hairs (setae) with barbed points situated in strategic areas that help to mix and macerate the food and has numerous ossicles (calcified structures) that serve as attachment sites for extrinsic muscles. Extrinsic and intrinsic muscles of the stomach cause the chamber to compress and expand, mixing the food with digestive enzymes. Interestingly enough, the digestive enzymes are not released directly into the cardiac chamber, but must travel backwards from the next chamber, or pyloric stomach.

Immediately before the pyloric stomach is a triangular structure consisting of a central tooth with a row of tooth-like denticles on either side (lateral teeth). This structure is known as the gastric mill and functions much like the gizzard of a turkey in that it is used to grind the food into fine particles. When the particles are fine enough, they pass into the pyloric stomach where they are filtered according to their size by ridges consisting of densely packed, feather-like hairs. Minute particles can pass into the midgut glands (the tomally) where they are further digested and absorbed into the hemolymph (blood). Material that is too large for the midgut gland is forced out of the filter and back into the pyloric stomach. From there it is passed into a straight, tubular portion of the midgut found both in the cephalothorax and continuing through the entire abdomen. Some of the end products of digestion are absorbed into the hemolymph by many small blood vessels that connect the midgut with the abdominal artery just above. The remaining material is packaged into fecal pellets surrounding by a mucous membrane. Contractions of the midgut force the pellets along to the hindgut and into the enlarged rectum. Rapid rectal contractions push the pellets out the anus at the base of the telson. (SEM micrograph by Kari Lavalli.)

Excretory System

Waste products from protein metabolism and tissue breakdown must be removed from an animal's body because they are usually toxic in nature and can upset ionic balances in the blood. The process of eliminating these wastes is known as excretion and occurs through excretory organs, which, in the lobster, are located at the bases of the antennae. They are called the green glands, because of their color, and consist of a glandular sac and a coiled tube that opens into a muscular bladder. It is from the nephropores at the base of the antennal segment that urine is released.

But metabolic waste products can also be eliminated via the gills, the digestive glands, the integument (or skin) which is lost when the animal sheds, and via phagocytes ("eating cells") in the gills.

Respiratory System

Twenty pairs of gills lie within two branchial chambers. The gills are of the trichobranch typeÑcomposed of numerous hair-like filaments arranged around a central rod. Water enters the branchial chambers through openings between the thoracic legs, passes up over the gills and out anteriorly (toward the lobster's head) in a current produced by the beating of the gill bailer, or scaphognathite, of the second maxillae. This current reverses every few minutes to clear the gills of debris and flush it out of the branchial chamber. The forward projecting nature of the "gill" current is important in that urine is released into it and projected forward. It is thought that the urine may contain important information about an animal's gender, sexual receptivity, physiological state (e.g., molt stage), etc.

BACK | CONTINUE