#scaphopoda

(Sources - 1, 2)

The class Scaphopoda is known more commonly for their shells, called “Tusk Shells” or “Tooth Shells.” They are the only class of molluscs to live exclusively below marine sediments. As such they are rarely seen alive, and even their shells are rarely found by beachcombers.

There are two orders of scaphopod: the Dentaliida (which may be paraphyletic) and the monophyletic Gadilida. Dentaliids are generally larger, have a shell which tapers uniformly, and their foot consists of one central and two lateral lobes which bends into the shell when retracted. They have strongly ribbed and rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze, making them harder to sense by animals which can detect electrical signals. On the other hand, gadilids are much smaller, have a shell whose widest portion is slightly posterior to its aperture, and have a foot which is disk-like and fringed with tentacles which inverts into itself when retracted. Gadilid shells are usually glassy-smooth and narrow, allowing them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators.

Scaphopod mantles are entirely hidden within their shell. Their foot extends from the larger end of the shell, and is used to burrow. Scaphopods position themselves head down in the substrate, with the tip of the shell projecting upward, but rarely above the level of 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 pull into the mouth. The mouth has a grinding radula and cartilaginous oral bolsters that break the bits into smaller pieces for digestion. In gadilids these are structured like zippers where the teeth actively crush the prey by opening and closing on it repeatedly. In dentaliids these work like a ratchet to pull prey into the esophagus, sometimes whole. Their prey are primarily single-celled organisms called foraminiferans, though some will also supplement their diet with plant matter. Scaphopods have no gills; the entire surface of the mantle cavity absorbs oxygen from the water. They also have no eyes, no osphradia, any or other distinct sensory organs. However, scaphopods do possess genes involved in photoreceptor formation and function implying scaphopods may have had eyes that degenerated over evolutionary time. Scaphopods have separate sexes, and external fertilisation. Eggs will hatch into free-living trochophore larvae, which develop into veliger larvae that more closely resemble the adults. Their three-lobed foot originates prior to metamorphosis while the cephalic tentacles develop post metamorphosis.

There is a good fossil record of scaphopods, with them emerging in the Mississippian (Early Carboniferous). The Ordovician Rhytiodentalium kentuckyensis has been interpreted as an early relative of scaphopods.

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Propaganda under the cut:

Polyplacophora. This class is made up of chitons, marine creatures that live under rocks. Their shells are made up of eight plates, protecting them while also allowing them to flex upwards or curl into a ball.

Scaphopoda. This class is made up of tusk shells, marine organisms that live in soft substrate offshore.

Apparently we have 30 of these at home, and I never drew one. So here is a Scaphopoda (Species unknown, probably collected in Egypt). 

Dentalium octangulatum

...is a species of scaphopod or tusk shell found throughout the western Indo-pacific. Like other scaphopods D.octangulatum is a suspension feeder and buries itself headfirst in the seabed and feeds on food items found in the substrate via a set of tentacles next to its mouth. D.octangulatum like all members of Dentalium has a mantle that fits entirely in the shell and a foot that extends from the shells larger end, this foot allows the scaphopod to burrow through the substrate.

Phylogeny

Animalia-Mollusca-Scaphopoda-Dentaliida-Dentaliidae-Dentalium-octangulatum