Explain how using an open circulatory system constrains the size of animals.

An open circulatory system constrain animals size because there is no motor that pump the hemolymph (hemolymph is like the blood in invertebrate animals), in a closed circulatory system is the heart that works as a motor to move the blood throught all over the body.

So the hemolymph can not travel through long distances what constrain the size of the animals that have open circulatory system.

thinking it would be physical characteristics

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.