Evolutionary Trends of Patterns in Digestion and Natural Selection in Animals

INTRODUCTION

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