What are the benefits and limitations of an open circulatory system?
Haemolymph is under relatively low hydrostatic pressure and therefore requires less energy expenditure.
No extensive system of blood vessels means less expensive to build and maintain.
May serve other functions too, e.g. to extend spider legs, and as a hytrostatic skeleton supporting the body in newly moulted aquatic arthropods.
However, low pressure means low flow rate, and without discrete continuous vessels, haemolymph cannot be directed towards tissues with high oxygen demand and CO2 buildup (except in some crustaceans). Most suitable for sedentary species that do not have high oxygen demands.