Tusk shell
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 Antalis vulgaris
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The tusk shells are a class of marine mollusks which vary in size from very small to medium sized. The official name of this class is Scaphopoda, meaning "shovel-footed".
These mollusks live on soft substrates offshore (usually not intertidally). Because of this subtidal habitat and their small size, many beachcombers are unfamiliar with them; their shells are usually not nearly as common or as easily visible in the beach drift as the shells of sea snails and clams. The class Bivalvia represents their nearest living relatives.
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[edit] Description of the shell
The shells of scaphopods are conical and curved in a planispiral way, and they are usually white in color. Because of these characters, the shell somewhat resembles a miniature elephant's tusk, hence the common name tusk shell. However, unlike an elephant's tusk, the shells of these mollusks are hollow and open at both ends; the opening at the larger end is the main or anterior aperture of the shell. The smaller opening is known as the apical aperture. Some tusk shells are minute, most are small, however, a few species reach 15 centimetres in length.
[edit] Habitat and distribution
Tusk shells live in the bottom sediment where they feed on microscopic detritus and organisms such as foraminifera. The several hundred known species of scaphopod are found in different areas throughout the world.
[edit] Anatomy of the soft parts
The mantle of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow through the substrate. A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food which they then convey to the mouth. The mouth has a grinding radula that breaks the bit into smaller pieces for digestion.
The scaphopod vascular system is rudimentary, lacking both heart and blood vessels; the blood is held in sinuses throughout the body cavity, and pumped by the rhythmic action of the foot.
[edit] The fossil record
Tusk shells are well-known in the fossil record, having first appeared in the Ordovician. As such they were the most recent of all molluscan classes to evolve. They are most closely related to the extinct molluscan class Rostroconcha. Among living creatures, their closest relatives are the Pelecypoda (bivalve molluscs).
[edit] Human use
The shells of Dentalium hexagonum, a scaphopod mollusk, were strung on thread and used by the natives of the Pacific Northwest as wampum, shell money.
Shells of the species Dentalium pretiosum were used as money in India.
[edit] References
- G. Steiner, "Phylogeny and classification of Scaphopoda", Journal of Molluscan Studies 58:385-400 (1992)
[edit] External links
- The Scaphopod Page - many further links
- Ron Shimek's scaphopod page
