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 Scaphopods have separate sexes, and external fertilisation. They have a single gonad occupying much of the posterior part of the body, and shed their gametes into the water through the nephridium. Once fertilized, the eggs hatch into a free-living trochophore larva, which develops into a veliger larva that more closely resembles the adult, but lacks the extreme elongation of the adult body. The three-lobed foot originates prior to metamorphosis while the cephalic tentacles develop post metamorphosis. Scaphopods remain univalved throughout their morphogenesis contrary to bivalves. Ecology Tusk shells live in seafloor sediment, feeding primarily on foraminiferans; some supplement this with vegetable matter. Classification The group is composed of two subtaxa, the Dentaliida (which may be paraphyletic) and the monophyletic Gadilida. The differences between the two orders is subtle and hinges on size and on details of the radula, shell, and foot. Specifically, the Dentaliids are the physically larger of the two families, and possess a shell that tapers uniformly from anterior (widest) to posterior (narrowest); they also have a foot which consists of one central and two lateral lobes and which bends into the shell when retracted. The Gadilids, on the other hand, 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 (in