Question

Only animals with a complete digestive system have evolved specialized organs for digestion. Why?

incomplete digestive system (2-way) - This system has a sac-like gastrovascular cavity (GVC) with only one opening to the environment. In this system, food passes through the mouth into the GVC where extracellular and intracellular chemical digestion takes place. The GVC extends throughout the body of the animal so that as food is absorbed it is available to the tissues; no transport system is needed. Waste products and food that has not been digested are expelled through the mouth. An incomplete digestive systems may be described as a “2-way gut” because materials pass in and out of the mouth. Incomplete digestive systems are found in the Cnidaria and Platyhelminthes. 2. complete digestive system (1-way) – This system has two openings to the environment: a mouth through which food enters and an @nus through which wastes are eliminated. After food enters the mouth, it travels through a tube that may have regions that are modified for specific digestive functions such as storage, mechanical breakdown, chemical breakdown or absorption. A complete digestive system may be described as a “1-way gut” since materials move one way through the system. Complete digestive systems are found in all the advanced animal phyla. Animals vary widely in the specializations of their digestive systems.

What are the advantages of a complete gut?

Answer 1

Primitive organisms have incomplete digestive system. They have simple body structure. Such organisms have only one opening which act as both mouth and anus. The food enter through this opening into gastrovascular cavity. The food is taken in and digested in cells of the cavity. Depending on the type of diet animal eat, digestive system has been evolved with specialised structures and glands to aid digestion of complex food. For example, ruminants such as cow They have stomach with four compartments; rumen, reticulum, omasum and abomasum (unlike human have single compartment stomach and eat cooked, easy food). They eat harsh and tough food. In order to digest such plant based food, special structures are required. They ferment food before mixing with acidic content.

Digestive system also evolved on the basis of digestive structure. Birds have relatively short and simple digestive system evolved to help in flying. Similarly, relatively short intestine and reduced ceacum (aid in plant based diet) has evolved in cats (carnivores).

Now simple organisms like sponges have incomplete digestive system. Their way of nutrition is simple. They are dependent upon small and single cells organisms present in water. They are easily taken up by cells of sponges body when water moves through it. Their diet is simple, so incomplete digestive system (simple) is present in such organisms.

Therfore, evolvution of digestive system depend upon type of food organism eat and complexity of body structure. Thus, helps animal in adaptation and survival according to environment.

One of the main advantage of complete digestive system is effiecient absorption of nutrients. Specialised organs together help in absorption of nutrients in best way. For example, long mall intestine with villi provide enough surface area for effiecient nutrient absorption. Many specialised organs work together to make best use of ingested food by dwaing out more nutrients.