Arthropodas have a complete digestive systems. They have different kinds of mouths depending on the species. Some can chew, some can suck objects. The digestive system is a tube that goes from the mouth to the anus. They have three stomachs, the foregut, the midgut and the hindgut.
Lobsters
The digestive system of the lobster consists of the three stomachs. The foregut contains a gastric mill, a set of grinding teeth that can grind food into fine particles. The particles then pass into the midgut glands where the particles are further digested. Material that is too large to be absorbed is eventually passed into the hindgut and then through to the enlarged rectum and out the anus at the tip of the lobster’s tail.
Spiders
When the spider's stomach's powerful muscles contract, the size of the stomach increases. This causes a strong sucking action that pulls the food through the stomach into the intestine. Juices in the digestive tube break the liquid food into molecules small enough to pass through the walls of the intestine into the blood. The food is then distributed to all parts of the body. Food is also pulled through the stomach into a cavity called the caeca. The ability to store food in the caeca enables spiders to go for long periods of time, over a year in some cases, without eating.
Crabs
It starts at the mouth. The mouth leads to an esophagus, which then leads to the stomach. There is an a chamber in the stomach called the gastric mill, where most fine food processing occurs. Nutrients are then allowed to pass through a filter and on to the midgut gland. The midgut is the major digestive organ in crabs, and performs numerous functions, including absorption of nutrients. Particles too large to pass through the filter and passed onto the intestine where they will be passed out of the animal through the anus as fecal material.