Respiration and Circulation

Since Mollusks have adapted to conquer both land and ocean habitats, different means to get Oxygen to the cells of the body have evolved. In Water-dwelling mollusks, gills used to take oxygen from the water can be found inside the mantle cavities. These gills are known as ctenidia or singularly, ctenidium. Landlubbing mollusks do not have ctenidia. Instead, their mantle is folded and contains many blood vessels. This structure is more similar to the human lung than the gills are. Inter-tidal mollusks are able to alternate between respiration in and out of water. Once in the bloodstream, the oxygen is carried throughout the mollusks' body by the heart. Some Species have on open circulatory system, meaning that blood does not always travel through special vessels. This style of Oxygen diffusion is adequete in sessile or slow mollusks like snails and clams, but more active groups of mollusks require a closed circulatory system. Examples of active mollusks with closed circulatory systems are squid, octopi, and cuttlefish.