Leeches
In leeches the digestive system starts with the jaws. In feeding they slice their way through the skin of the host. Behind the blades is the mouth, located ventrally at the anterior end of the body. It leads into the pharynx, then the esophagus, the crop, the gizzard, and the intestinum, which ends at the posterior sucker. The crop is an extension of the alimentary canal that functions as an expandable storage compartment. Some kinds that live on small invertebrates have neither proboscis nor jaws, but engulf their food with the mouth. The leech produces an anticoagulant that prevents the stored blood from clotting, plus other agents that inhibit microbial decay of the blood. All leech species studied do produce endogenous intestinal exopeptidases which can unlink amino acids. Deficiency of digestive enzymes but, more importantly, deficiency of vitamins, is compensated for by enzymes and vitamins produced by endosymbiotic microflora.
Earthworm
The essential stomach of the earthworm is a straight tube which extends from the mouth to the anus. It is differentiated into a buccal cavity, pharynx, esophagus, crop, gizzard and intestine. Food enters the mouth. The pharynx acts as a suction pump, drawing in food. In the pharynx, the pharyngeal glands secrete mucus. Food moves into the esophagus, where calcium from the blood and previous meals, is pumped in to maintain proper blood calcium levels in the food pH. In the gizzard, muscular contractions crush the food with the help of mineral particles ingested along with the food. Food continues through the intestine for digestion. The intestine secretes pepsin to digest proteins, amylase to digest polysaccharides, cellulase to digest cellulose, and lipase to digest fats. Instead of being coiled like a mammal intestine, an earthworm's intestine increases surface area to increase nutrient absorption by having many coils running along its length.
Polychaete Worms
The mouth of polychaetes is located on the peristomium, the segment behind the prostomium, and varies in form depending on their diet, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. It possesses a pair of jaws, formed from sclerotised collagen, and a pharynx that can be quickly everted, allowing the worm to grab food and pull it into the mouth. The digestive tract is a simple tube, usually with a stomach part way along. See diagram below for more details.