| MadSci Network: General Biology |
I'll start with the lower life forms and work my way up.
Eubacteria and protozoans have no need for a 'circulatory system' as they are single celled organisms. All nutrients diffuse through the cellular membrane or are carried across by active transport.
Poriferans (sponges) are simple multicellular organisms. However, they do not specifically have an internal circulatory system. Rather, ciliated cells continually move seawater throughout the sponge so other members of the colony can derive nutrients and oxygen from the water.
Annelids have a closed circulatory system. Unlike poriferans, they are more than 2 cell layers thick, so nutrients cannot be distributed to all cells in the organism by means such as diffusion and osmosis, or by moving external fluids (seawater) to sites more internal. The circulatory system consists of tubes that extend along the length of the worm. Endolymph flows within these vessels and distributes oxygen and other factors throughout the body of the worm.