Unlike plants, animals cannot prepare their own food. Instead, they procure food by feeding on other organisms.
Hence, they are called Heterotrophs. Heterotrophic nutrition is of two types. Saprozoic and Holozoic.
Saprozoic nutrition in very primitive animals, especially some protozoans, and parasites exhibit saprozoic mode of nutrition. They absorb food materials dissolved in water through their body surfaces.
Some animals secrete enzymes into the external medium. These enzymes breakdown the complex organic molecutes into simple organic molecules. These are then absorbed by the animal through its body surface. As the food is already digested and in the form of small molecules, some of these animals may not have special digestive organs. Even if present, the digestive system in these animals is very simple.
Holozoic nutrition Majority of animals take solid particulate food or liquid food through special feeding mechanisms and digest them with the help of digestive enzymes. This mode of nutrition is know as holozoic nutrition.
The size of food particles varies considerably ranging from microscopic organisms to large animals and plants. Depending on the size of the food, different mechanisms to capture the food.
Some animals use pseudopodia (Amoeba), tentacles (Hydra) for capture the food. These animals do not cut the food into small pieces and swallow the entire food at one time. Other animals which feed on solid food have mechanisms to cut the food into small pieces before they consume it.
For example, snails have snarp teeth like structures on their tongue called Radula with which they scrape the surface of food and swallow the food particles.
Some animals, like lions and tigers, are equipped with organs to chase, seize (capture) kill, cut and cat the prey.
Some animals feed only on liquid food. For example scorpions and spiders kill the prey and inject digestive enzymes into the body of the dead animal. These enzymes digest the food and convert it into liquid form which the animal will feed.
Houseflies, butterflies, mosquitoes, aphids, leeches etc.,also feed on liquid food. Butterflies have long tube like proboscis which is inserted into the flowers and nectar is sucked.
Mosquitoes have special organs to pierce through the skin and suck the blood. Blood sucking animals such as mosquitoes and leeches secrete substances into the blood to prevent blood from clothing while they are feeding.
Animals like earth worms feed on soil containing decomposed organic material.
Thus,a variety of feeding mechanisms are seen in animal kingdom and the type of feeding mechanisms in the animal depends on the type and nature of the food.
Mono and Polyphagous animals:
Some animals feed on only one type of food material and these are called monophagous animals.
A typical example for this is the caterpillar larva of silkworm which feeds only on mulberry leaves.
Several animals feed on plant material are called herbivores while those feeding on other animals are called carnivores. Animals which eat both plants and animals are called omnivores.
Diet of an animal may be different in different sexes or at different stages of its life. For example, male mosquito feeds on plant juices white female mosquito feeds on blood. Similarly, caterpillar larva feeds on leaves while the butterfly feeds on the nectar.
Thus, there is a great variation in the type of food consumed and the method by which it is consumed in holozoic animals.
