Resembling outsize crabs with their skeleton on the outside, Melfans are warm-blooded amphibians who are as happy scuttling about the bottom of pools and lakes as they are on land. The Melfan "shell" is a thick, low-slung, osseous carapace with beautiful, rich patterns used in courtship. It is filled with a shock-absorbent fluid in which their organs are suspended, loosely tethered to the outside; this fluid needs draining for surgery, which must be performed quickly before the organs' unsupported weight causes damage.
The Melfan head has large, vertically lidded eyes, enormous mandibles, and pincers projecting forward from the place where ears should have been. There are also two long, thin, fragile, flexible feelers growing from the sides of the mouth, which feel like soft, furry feathers to the touch. When angry they click their pincers together rapidly; a slow clicking indicates a desire for attention. Projecting from slits where the bony carapace and underside join are six thin, bony, tubular, multijointed limbs, all capable of manipulation and feeling like warm, slightly damp logs.
Melfans eat every ten hours, and sleep for four hours in between. Favourite dishes include swamp nuts and live crottled greeps, although at Sector General they have to make do with moving electrically charged substitutes. They are intensely emotional, passionate people who wear no clothing and relax in furniture resembling surrealist wastepaper baskets. They are immobilised if they get turned onto their backs.