Original sentence: Some tusk shells are minute, most are small, however, a few species reach 15 centimetres in length
Edited sentence: A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself.A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body.Instead, dexoygenated water is expelled rapidly back through the apical aperture through muscular action once ever ten to twelve minutes.Metabolic waste is excreted through a pair of nephridia close to the anus.Once fertilised, 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.One pair each of cerebral and pleural ganglia lie close to the oesophagus, and effectively form the animal's brain.Scaphopods have no eyes, or other distinct sensory organs.Scaphopods have separate sexes, and external fertilisation.Some tusk shells are minute, and most are  long; however, a few species reach  in length.The anus opens on the underside of the animal, roughly in the middle of the mantle cavity.The nervous system is generally similar to that of gastropods.The remainder of the digestive system consists of an oesophagus, stomach, and intestine.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 up into the water.There are no gills; the entire surface of the mantle cavity absorbs oxygen from the water.They have a single gonad occupying much of the posterior part of the body, and shed their gametes into the water through the nephridium.Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream.Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by cilia