Evolutionary Trends of Patterns in Digestion and Natural Selection in Animals

Digestion of food materials is not required for autotrophs i.e. organisms having Chlorophyll, because they need not to produce simple food after breaking down the complex food. But, most of the animals are heterotrophs. So they need to digest the complex food materials to make them simple.
In Animals digestion may occur outside or inside the cellular components. If digestion takes place inside the cell then it would be termed as Intracellular digestion otherwise if digestion takes place outside the body cell then it would be termed as Extracellular digestion.

DIGESTION IN PROTOZOANS

In subkingdom Protozoa, animal body is made up of a single cell only. So, there is no chance of performing digestion in outside the body cell, because a cell means one animal body to Protozoa ! Therfore the mode of digestion in Protozoa is always intracellular (except Euglena and Chrysamoeba, because they are autotrophs). During intracellular digestion, food particles or liquid foods are ingested into a cell by Phagocytosis and Pinocytosis processes respectively. Then the food materials are enclosed within food vacuoles that fuses with secondary Lysosomes and foods are broken down by the enzymes secreted from Lysosomes and also by Hydrochloric acids.

DIGESTION IN PORIFERANS

In Poriferans like Sponges, the process of digestion is more complex than the Protozoa, still the intracellular digestion takes place. In their body a well formed canal system is present, which may be of Ascon, Sycon, Leucon or Rhagon categories. Water current produced by flagella bears food particles are passed into their bodies by numerous small openings called Ostia. Collar cells take the food particles from the water current. Within the Collar cells food materials are digested by intracellular process by the help of Phagocytosis and Lysosomes. After digestion the undigested materials are removed out from a large pore called Osculum.

DIGESTION IN CNIDERIANS

In cniderians like Hydra the type of digestion is an intermediate between Intracellular and extracellular process. This conjunctive category is also found in Phylum Ctenophora and Phylum Platyhelminthes. When a Hydra takes food materials inside the coelenteron or gastrovascular cavity (food cavity) then food materials are partially digested. But when these semidigested food materials are taken inside the cellular materials of the gastrovascular cavity then the complete breakdown takes place.

DIGESTION IN HIGHER ANIMAL PHYLUMS

In most of the animal phylums like Chordata food materials are digested by Extracellular process i.e. digestion takes place outside the body cell. In their bodies there are digestive systems where foods are digested by the help of different types of enzymes. Besides chordates extracellular digestion is also found in annelids, arthropods etc.
NOTE : To see the different steps of digestion in Human I would like to refer you to my another article 'The Roles of Different Enzymes in the Digestion Within the Human Alimentary canal : a Guide for Students' published in Associated Content.

DISCUSSION

It was seen from the above evidences that the patterns of digestion are moved towards the extracellular category along with the evolutionary progress. As the complexity is grown in the body structures as well as in the digestive system of the organisms the necessity of intracellular digestion is decreased gradually and the extracellular digestion procedure is preferably selected by the nature.

In subkingdom Protozoa, the bodies of the organisms are made up of a single cell only. So, digestion is done within their cell and there is no chance of performing digestion in outside their body cells. In the phylum Porifera digestion is also intracellular because in their simple body structure there is only a chance of digestion of foods by Phagocytosis.

But as the tree of evolution is further grown and the cnidarians have come then there is a sign of complexity generation is observed as the intermediate type of digestion is originated. In cnidarians the trace of starting the extracellular digestion is observed. As per the records of evolutionary history the further development of animals occurred after cnidarians that shows more body complexity. The developed animals have their complex body structures with a well developed digestive tract, where digestion occurs outside the cell and after digestion absorbed food materials are assimilated within the protoplasm of the cell for generating energy by Glycolysis and Kreb's cycle i.e. the respiration. Therefore I have established from my whole discussion that natural selection has preferred the extracellular digestion for the further development of the evolutionary status of the animals.