Shells The shells of the members of the Gadilida are usually glassy-smooth and narrow, with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators. The Dentalids, by contrast, tend to have strongly ribbed and rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze. This makes them harder to detect by animals such as
ratfish, which can sense the electrical signals given off by the most minute muscle movement.
Mantle 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. The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting upward. This end seldom appears above the level of the substrate, however, as doing so exposes the animal to numerous predators. Most adult scaphopods live their lives entirely buried within the substrate. Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by
cilia. There are no
gills; the entire surface of the mantle cavity absorbs
oxygen from the water. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.
Feeding and digestion '' 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 radulae and cartilaginous oral bolsters of the Gadilidae are structured like zippers where the teeth actively crush the prey by opening and closing on it repeatedly, while the radulae and bolsters of the Dentaliidae work rachet-like to pull the prey into the esophagus, sometimes whole. The massive radula of the scaphopods is the largest such organ relative to body size of any mollusc (among whom, except for the
bivalves, the presence of which is a defining characteristic). The remainder of the digestive system consists of a digestive
diverticulum,
esophagus,
stomach, and
intestine. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The
anus opens on the ventral/ underside of the animal, roughly in the middle of the mantle cavity.
Vascular system The scaphopod vascular system is rudimentary lacking heart
auricles as well as corresponding
ctenidia (gills) and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. The
heart, a characteristic feature of all other groups of mollusca, has been considered totally lost or reduced to a thin fold of the
pericardium; however, according to more recent studies, the muscular, regularly beating
perianal blood sinus is
homologous to the
ventricle and is therefore considered the scaphopod heart. Metabolic waste is excreted through a pair of
nephridia close to the anus. The tusk shells appear to be the only extant molluscs which completely lack the otherwise standard molluscan
reno-pericardial apertures. Furthermore, they also appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water (through two "water pores" located near the nephridial openings). These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.
Nervous system The nervous system is generally similar to that of
cephalopods. One pair each of cerebral and pleural
ganglia lie close to the oesophagus, and effectively form the animal's
brain. A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body, and connect to pavilion ganglia via long connectives. Radular and sub-radular ganglia are also present, as are
statocysts with
staticonia. Scaphopods have no
eyes, no
osphradia, 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. ==Reproduction and development==