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The
Class Scaphopoda contains about 400 species of molluscs called tooth
or tusk shells, all of which are marine. Scaphopods range in size
from 4 to 25 cm long (although most species are 2.5-5 cm in
length). In comparison to other molluscs, scaphopods are a "young"
group, appearing in the fossil record about 450 million years ago
during the Ordovician period, and present evidence suggests that
they had the same ancestor as the bivalves.
Tooth
and tusk shells live sedentary lives buried in sand or mud
substrates, mostly in water as deep as 6 km. Their shells grow
linearly as a hollow, curved tube with an opening at each end; water
enters and leaves the narrower end, which protrudes above the
substrate. Unlike many molluscs, scaphopods lack ctenidia (gills),
a heart and a circulatory system, and blood circulates through the
various sinuses of the hemocoel as a consequence of the foot's
rhythmic movements. Small food particles (such as foraminiferans)
from the surrounding sediment are captured by 100-200 sticky
tentacles called captacula, which are then transported to the mouth. |
