Scaphopoda

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Scaphopoda

a class of primitive molluscs, possessing a tube-like shell open at both ends.
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The scaphopods (or "tusk shells") burrow into sea-floor sediment using their muscular "foot" and live head-down.
For instance, in bivalves and scaphopods the cleavage pattern of the 2d cell, the first somatoblast, completely differs from the cleavage pattern of 2d in gastropods and polyplacophores.
On the basis of their presence or absence in dredge samples, the 26 species of marine molluscs recovered from Expedition Fiord (17 bivalves, 8 gastropods, and 1 scaphopod) cluster into two discrete mollusc associations.
In a scaphopod (Wanninger and Haszprunar, 2003) and in bivalves (Matsutani and Nomura, 1986; Croll et al., 1995), the labeled cells are also uniformly localized in pedal, cerebral, and other peripheral ganglia.
Embryos of a polyplacophoran (Acanthochiton) and a scaphopod (Dentalium) showed an expression pattern completely comparable with that in Patella (A.
25), as well as the scaphopod Dentalium vulgare (Dufresne-Dube et al., 1983) but is unlike the acrosome reaction and sperm entry described for other archaeogastropods (Lewis et al., 1980; Shiroya and Sakai, 1984; Azevedo et al., 1985).
A duo-gland adhesive system has also been described, recently, in the captacula (i.e., the food-collecting tentacles) of scaphopod mollusks (Byrum and Ruppert, 1994), still widening the distribution range of this adhesive system in marine invertebrates.
An inferred phylogeny for molluscan classes that puts gastropods, bivalves, and scaphopods in a monophyletic group (Kocot, 2013) reduces the implied number of losses of a metatroch if a metatroch originated in the ancestor of that clade; but implied losses still outnumber gains because numerous clades within the Bivalvia and Gastropoda lack metatrochs.