What's the difference between thallus, holdfast and sessile colony?

Thallus, holdfast and sessile colonies are used to describe algal colonies.
Thallus refers to a primitive plant like body that is not divided into root, stem and leaves.
Holdfast is a special structure that is used for anchorage to the substratum.
Sessile colonies are such colonies that uses holdfast to attach itself to the substratum.

Plants take energy from the sunlight with the of chlorophyll so the plant starts the food chain. then an herbivore eats the plant and gets its energy.then a carnivore or a human eats the herbivore and gets its energy. : )