Living, animated, and unliving things can be categorized by
physiology, which groups species by the biological or (non-biological)
mechanisms of life (or animation). These are the broad terms by which
magic works; there is no correct word for the 'insect' category for
example, which includes spiders and some varieties of unnatural
horror, nor for 'fish' which includes tiny squid and mind-bending
krakken.

  Animal: mammals, birds and reptiles. These normally have a head,
    limbs, distinct organs including muscles, breathe air, and circulate
    blood.
  Fish: living things that can breathe water, and normally have
    circulation, nerves, and muscles.
  Insect: living things which normally circulate ichor and have neither
    muscles nor an internal calcium skeleton.
  Plant: living things which circulate sap, have neither nerves,
    muscles, nor bones, and normally without many specialized internal
    organs.
  Non-flesh: things which are not born, not budded, nor hatched, but are
    crafted or found. Closer to clocks or fire than life.
  Undead: dead flesh, magically re-animated.

Beings can have one, several or none of these classifications. If a
healing spell's power will be modified by physiology, the lowest
non-zero healing value is used.
    
See also: help skill physiology, help metaraces