ABSTRACT
The polyplacophoran circulatory system is mostly an open system and, besides the pumping of the heart, body movement also plays a role in assisting blood flow. Most molluscs have an open circulatory system. The circulatory and excretory systems are thus linked, and they are also integrated with the respiratory surfaces where gases are exchanged, and with the digestive system, which supplies the nutrients. Body volume and osmoregulation in molluscs is controlled in part by hormones. Fresh-water molluscs must extract ions from the surrounding water against an electrochemical gradient. Cephalopod excretory systems diverge considerably from those of other molluscs and involve several distinct organs and tissues with different functions. Gastropod excretory systems are highly variable, especially in the form and function of the kidney(s). The excretory system of hygrophilans is similar to that of terrestrial stylommatophorans with additional adaptations to their fresh-water life.
