The digestive tube runs longitudinally in the center of the body cavity, passes through the diaphragm, and the muscular layer of the tube wall is developed, which can improve peristalsis and digestive function. The digestive tract differentiates into parts such as the mouth, mouth, throat, esophagus, sand sac, stomach, intestine, and anus. The mouth can be turned out of the mouth and food is ingested. The pharynx is muscular, the muscles contract, and the pharyngeal cavity is enlarged to aid feeding. There are single-celled pharyngeal glands outside the pharynx that secrete mucus and proteases, have moist food, and have initial digestive effects. The posterior pharynx is connected to a short, thin esophagus, which has esophageal glands on its walls, which secrete calcium and neutralize acidic substances. Behind the esophagus is a muscular sand sac (gizzard) lined with a thick cornea that grinds food. From the mouth to the sand sac is formed by the ectoderm, which belongs to the foregust. A segment of the digestive duct after the sand sac is rich in microvascular, multi-glandular, called the stomach. There is a ring of gastric glands in front of the stomach, which function like pharyngeal glands. About the 15th body segment after the stomach, the digestive tube enlarges to form the intestine, and its dorsal central recess is recessed into a blind channel (typhlosole), which increases the area of digestion and absorption. Digestive and absorption functions are mainly carried out in the intestine. The outermost layer of the intestinal wall of the visceral membrane is specialized into yellow cells. Beginning in the 26th body segment, a pair of cone-shaped cecums (caeca) protrude forward on both sides of the intestine, which secrete a variety of enzymes and are important digestive glands. The stomach and intestines are derived from the endoderm and belong to the midgut. The hindgut is short, accounting for more than 20 body segments at the back of the digestive tube, with no blind passage and no digestive function. Use the opening outside the body. The digestive system of earthworms is composed of more developed digestive ducts and digestive glands, which are composed of the mouth, pharynx, esophagus, sac, sand sac, stomach, small intestine, cecum, rectum and anus.
The mouth and pharynx form the feeding route of the earthworm, and the mouth is the expanded part of the inside of the mouth, which is shorter, located on the ventral side of the perioral sac, and occupies only the second or first to second body segments. The wall of the cavity is very thin, there are no jaws and teeth in the cavity, and it is impossible to chew food, but it has the effect of receiving and sucking food.
The mouth is followed by the pharynx, and the pharyngeal wall has a more developed muscle layer, which extends backwards to about the sixth body node. The lining of the mouth and the epithelium of the pharynx are covered with a stratum corneum. There are many radial muscles on the outside of the pharynx that are connected to the body wall, and the expansion or contraction of the pharyngeal cavity or valgus is done by the contraction of the muscles, which is convenient for earthworms to eat. Therefore, earthworms generally like to swallow moist, soft food, while dry, large and hard food is more difficult to eat. Some large terrestrial earthworms, such as those of the genus Ring hair and heterolibis, have a gray-white, lobed gland on the dorsal wall of the pharynx, the pharynx gland, which secretes digestive juices containing protease and amylase. It can be seen that in addition to the function of feeding and storing food, the pharynx also has a digestive effect.
The back of the pharynx is a narrow and long tubular esophagus, where there are a pair or pairs of calcium glands located on both sides of the esophagus, which is a gland formed by the inverted wall of the esophagus, which secretes calcium to reduce the excess calcium that enters the body with food, and by controlling the concentration of ions to maintain the acid-base balance of body fluids and blood.
The sac is an enlarged thin-walled sac behind the esophagus. It has the function of temporarily storing and moisturizing, softening food, and also has a certain filtering effect, and can also digest some protein. Some species lack sacs and sand sacs.
The sac is followed by a hard, spherical or oval sand sac, the so-called "stomach." Some earthworms have only 1 sand sac and occupy 1 or more body segments. Usually terrestrial earthworms have sand sacs. The sand sac has an extremely well-developed muscular wall, the inner wall of which has a hard stratum corneum. There are often sand particles in the sand sac cavity, because the muscles of the sand sac contract and squirm, the food can be continuously squeezed, coupled with the hard cornea and the grinding of the sand grain, the food will gradually become smaller, broken, and finally become a slurry chyme, easy to absorb. The presence of sand sacs is the result of earthworms adapting to life in the soil. After the sand sac, there is a narrow, multi-glandular tube called the stomach (surrounded by the heart and seminal vesicles, sometimes called the small intestine). Because there are glands on the stomach wall that can secrete amylase and proteases, the stomach is an important digestive organ of earthworms.
Immediately after the stomach is a large and long digestive tube called the small intestine, sometimes called the large intestine, which is a large and long digestive tube. The tube wall is thinner, the outermost layer is the peritoneal visceral layer formed by yellow cells, the outer part of the middle layer is the longitudinal layer, the medial side is the ring muscle layer, and the innermost layer is the small intestinal epithelium. These epithelial cells are composed of granular and vacuole-rich secretory cells and long, cone-shaped digestive cells that secrete digestive juices containing a variety of enzymes and absorb digested nutrients. The small intestine is recessed along the dorsal midline to form a blind passage, which aids digestion and absorption of the small intestine. Most food is digested and absorbed in the intestine.
The narrow and thin-walled part of the posterior end of the small intestine is the rectum, which generally has no digestive effect, and its function is to turn the digested and absorbed food residue into worm feces through this to the anus and excrete the body. (Image source: CFP)