How do planarians survive without a circulatory system?

Concepts to Consider: Planarians are usually fairly small animals. The small size maximizes the ratio of surface area to body volume, which maximizes diffusion. Maximizing surface area is also accomplished through the flat body shape of these animals. Internally, diffusion of body fluids to various regions of the body is accomplished through a highly branched gut, which maximizes the surface area with which it comes in contact. Combined, these features allow for efficient gas exchange, waste removal, and the delivery of nutrients, which are some of the functions carried out by a circulatory system.