Morphology

In concurrent exchange, the flow of the water is in the same direction as the flow of hemolymph. Hemolymph entering the ctenidia lacks oxygen, whereas the water across from it is fully saturated with oxygen. As oxygen diffuses across the gill the hemocyanin entering the body is only 50% saturated with oxygen. In counter current exchange, the flow of water opposes that of hemolymph in the ctenidia. This maximizes the diffusion gradients of both oxygen and carbon dioxide and is much more efficient.

General Anatomy

Chitons belong to the Phylum Mollusca which is characterized by animals with bilateral symmetry, large complex kidneys, a mantle with shell glands that secrete calcareous epidermal spicules, shell plates or shells, a muscular foot, and a dorsally concentrated visceral mass containing the organs for digestion, circulation, reproduction, and respiration. Molluscs are primarily divided into classes based on modifications to the foot and shell. Chitons or those species within the Class Polyplacophora are flattened and elongated. They typically have eight shells or valves inside of a flexible mantle or girdle. The shells are composed of three layers, the first layer being the outer periostracum, a thin delicate organic membrane. The second layer is the colored tegmentum which is composed of organic material and calcium carbonate subfused with pigments. The bottom most layer is the calcareous layer or articulamentum, a thick calcareous pearly layer unique to chitons.

Lacking eyes, chitons are able to sense the environment with both sensory structures in the girdle as well as photosensory organs (aesthetes) in their shells. The colored tegmentum layer of the shell is penetrated by vertical canals that lead to megalopores and micropores that are able to sense light. This is why they are so difficult to pry off of rocks. Even before you have touched them, they know that something has passed over their shell. Roll the mouse over the images below to learn about their morphology.

Circulation/Gas Exchange

Chitons have an open circulatory system. While all vertebrates (save some Antarctic fish) and many invertebrates use hemoglobin, an iron based metalloprotein, for oxygen transport, species within the Class Polyplacophora have hemocyanin, a copper based metalloprotein in its blood (Herskovits, 1987). As a result, blood within the arteries that is oxygenated is a brilliant blue color and deoxygenated blood in the veins is clear. Hemocyanin is about one-quarter as efficient as transporting oxygen as hemoglobin. Their blood or hemolymph is pumped by a dorsally located heart within the pericardial chamber.

Gas exchange occurs over gills or ctenidia. Water is carried over the ctenidia with the beating of cilia. Water is carried across the gill filament in the opposite direction to the flow of hemolymph, called counter current exchange, to make the uptake of oxygen more efficient.