Animals of the Phylum Mollusca typically have a shell, a muscular foot, a head with a mouth, sensory organs and a visceral mass, which contains the gut, heart, gonads and nephridia. The body wall is folded enclosing the body in the mantle. The mantle cavity within the mantle consists of an enclosed water space; it is here that the gills are located. Mollusks have a reduced coelom that contains the heart. Chitons (class Polyplacophora) are oval shaped animals that are dorsoventrally flattened. The dorsal shell consists of eight plates called valves. Chitons use their tough, oval foot for attachment to rocky substrates.

External Anatomy

Dorsal Surface/Shell
The dorsal side of the animal is oval shaped, arched (convex) and is known as the mantle. It is the mantle that secrets the shell. The shell of Katharina tunicata is composed of eight overlapping valves or plates composed of calcium. Each of these valves are arched and firmly embedded in the girdle, leaving only the midsection of the plates exposed. The entire girdle is covered with microscopic spicules composed of either silicon or calcium. (In many of the animals I found it appeared the valves were so embedded in the girdle that it appeared as if the girdle was growing over them.) The valves are comprised of two layers, the tegmentum which is a thick, dark, moderately soft layer that covers the exposed portion of the valve and the inner layer, called the articulamentum. Frequently, the articulamentum can be seen where the tegmentum has naturally been worn away.

Ventral Surface
The muscular foot is located on the ventral surface. The foot is separated from the girdle by the mantle cavity. Within the mantle cavity are several (50-60) rows of gills. The ciliated epithelium keeps water circulating over the gills. This epithelium contains mucus-secreting cells but does not serve as a cuticle. The gills are attached to the roof of the mantle cavity and hang down within the cavity. The posterior end of the animal consists of the anus, which is found on the top of the mantle cavity.

Internal Anatomy
The inner cavity of K. tunicata, called the hemocoel houses most of the internal organs of the animal. The hemocoel is an immense system of unlined sinuses. In the center of the hemocoel are orange colored gonads, which lie on top of the digestive system, and the heart, which is enclosed within the pericardium (coelom). The pericardium is a fluid filled space at the posterior end of the animal. It extends the length of the hemocoel and fills up the space below the last two plates. The gonads take up most of the space of the hemocoel. At the posterior end of the animal are two gonoducts, which open to the exterior of the animal through gonopore of the mantle cavity. The excretory system consists of two metanephridia, which extend the length of the hemocoel. Located between the mouth and anus is a coiled digestive tube. The mouth, located at the center of the head, houses the radula, a scraping organ used in feeding. The radula is composed of rows of black chitionous teeth that are coated with magnetite.

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