Question

How can some small multicellular organisms function without a closed circulatory systems?

Answer 1

Metabolism

Explanation:

An open circulatory system is adequate for slow metabolism, as it generates very low values of blood pressure. These animals are not very active, and sometimes they don't even use blood as oxygen transport (insects have a very well developed respiratory system) but as a way of motility, for example, spiders build up pressure in the limbs they want to extend.