Tusk shells are marine molluscs that have well identified fossils which first appear in the Mississippian period. They are, therefore, the most recent of all the molluscan classes to evolve.
Tusk shells generally range between 0.5 centimeter (0.2 inch) and 15 centimeters (6 inches) in length, though species from the order Dentaliida are usually significantly larger than species from the order Gadilida. They have a conical, slightly curved shell that is normally white in color, and hollow and open at both ends. The opening at the larger end is known as the main or anterior aperture, and the opening at the smaller end is called the apical aperture.
Tusk shells use a foot, extended from the wider end of the shell, to burrow into the sediment. They position themselves with the rear end of their body (apical end of the shell) projecting up into the water. The mantle of a tusk shell is entirely within its shell. Water enters the mantle cavity via the apical aperture and is wafted along the body surface by the cilia. Tusk shells do not have any gills and the entire surface of the mantle cavity can absorb oxygen from the water. Unlike other molluscs, they do not have a continuous flow of water going over them with a separate exhalant stream; instead every 10 to 12 minutes, deoxygenated water is rapidly expelled back through the apical aperture.
Located around the foot are a number of minute tentacles called captacula, which are used to sift through sediment and capture food, and then convey it to the mouth. In the mouth the food is ground up by the radula so that it can be digested. The digestive system consists of an esophagus, a stomach, and an intestine. When the food reaches the stomach a digestive gland secretes enzymes into the stomach to aid digestion. On the underside of the tusk shell, situated in the middle of the mantle cavity, is the anus.
Tusk shells lack a heart and blood vessels and have only a rudimentary vascular system. The blood is held in sinuses throughout the body cavity and is pumped around the rhythmic action of the foot. Tusk shells have no eyes or any other distinct sensory organs.
Distribution and habitat
Tusk shells are found in oceans throughout the world and inhabit the soft, muddy bottom sediment of coastal waters down to depths of 7,000 m (22,966 ft).
Tusk shells have separate sexes and when mating, external fertilization occurs. Both eggs and sperm are released into the water and fertilization takes place. When the eggs hatch the youngsters are free living trochophore larvae which later develop into veliger larvae before they become fully mature adults.
Diet and methods of capturing prey
Tusk shells feed on detritus, foraminifera, and microscopic animals.
Tusk shells, or scaphopods, belong to a class of marine molluscs known as Scaphopoda. Within this they are divided into two orders: Dentaliida and Gadilida. There is little difference between the two orders, and they are classified by details of their shell and radula.
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