Animal Digestive Systems: A Comprehensive Look
Animal Digestive Systems
In animals, the primary function of the digestive system is to transform food into usable molecules for body cells to perform their vital activity (cell metabolism). Key substances that should be incorporated in an animal’s food include:
- Carbon-based molecules such as carbohydrates and lipids
- Nitrogen-based molecules (proteins)
- Vitamins
- Minerals and water
The Digestive Process
The digestive function consists of four main processes:
- Capture and Ingestion: The type of ingestion an animal uses is related to its physical structure.
- Digestion: This involves the breakdown of complex organic compounds in ingested food into simpler substances that the body can absorb, using hydrolytic enzymes. Digestion can be:
- Intracellular: Takes place inside the cells, without the need for gastric cavities.
- Extracellular: Occurs gradually in a well-coordinated process.
- Mixed: Begins as extracellular but finishes inside certain cells within the body.
- Absorption: Simple molecules obtained during digestion enter the animal’s bloodstream from inside the digestive tract to be distributed throughout the body.
- Egestion: The expulsion of unusable remains from the body after digestion.
Types of Digestive Systems in Invertebrates
Sponges
Sponges possess specialized cells called choanocytes, which line the inner cavity of the animal’s body. Food particles in the water are captured by the choanocytes in digestive vacuoles. After digestion, other cells, called amebocytes, carry these substances to other cells of the body.
Annelids
Annelids possess a digestive tract with a very muscular pharynx. They have a crop to store food and a gizzard with small sand grains for shredding. Digestion and absorption occur in the intestine, and waste is expelled through the anus. Extracellular digestion is performed along the tube.
Cnidarians
Cnidarians have tentacles around the mouth to introduce nutritious particles. These tentacles contain numerous offensive-defensive cells called cnidoblasts. When food reaches the gastrovascular cavity, enzyme-secreting cells lining the inner wall of the cavity start their secretion and perform extracellular digestion. Partially digested particles are then phagocytosed by other cells of the cavity, where intracellular digestion takes place. Therefore, digestion in these animals is mixed.
Molluscs
Most molluscs, except for lamellibranchs, possess a digestive system composed of a mouth fitted with a radula, esophagus, stomach, and intestine leading to the anus. They have a gland called the hepatopancreas that aids in the digestive process. Digestion is extracellular. Lamellibranchs have a filtering system.
Arthropods
Arthropods exhibit a rich variety of structures for the capture and ingestion of food, as well as different types of feeding habits. In insects, for example, there are several types of oral appliances: chewing, sucking, piercing, etc. In arthropods, the mouth is surrounded by appendages responsible for the capture and ingestion of food. After the mouth, there is the pharynx, esophagus, midgut, and hindgut leading to the anus.
Echinoderms
The gastrointestinal tract in echinoderms begins in the mouth, which is located ventrally and may or may not have a masticatory device or skeletal file known as Aristotle’s lantern, which is formed by five jaws. They have a short esophagus and a large stomach. The intestine is small and ends in an anal pore.