of degree of the same thing. Look. You have
a kind of crustacean living in the lakes here, very much like an
ordinary crab. It has large claws in which it holds anemones,
tentacled sea animals with no power of motion. The crustacean waves
these around to gather food, and eats the pieces they capture that
are too big for them. This is biontergasy, two creatures living and
working together, yet each capable of existing alone.

"Now, this same crustacean has a parasite living under its shell, a
degenerated form of a snail that has lost all powers of movement. A
true parasite that takes food from its host's body and gives nothing
in return. Inside this snail's gut there is a protozoan that lives
off the snail's ingested food. Yet this little organism is not a
parasite, as you might think at first, but a symbiote. It takes food
from the snail, but at the same time it secretes a chemical that
aids the snail's digestion of the food. Do you get the picture?
All these life forms exist in a complicated interdependence."

Brion frowned in concentration, sipping at the drink. "It's making
some kind of sense now. Symbiosis, parasitism and all the rest are
just ways of describing variations of the same basic process of
living together. And there is probably a grading and shading between
some of these that make the exact relationship hard to define."

"Precisely. Existence is so difficult on this world that the
competing forms have almost died out. There are still a few left,
preying off the others. It was the cooperating and interdependent
life forms that really won out in the race for survival. I say life
forms with intent. The creatures here are mostly a mixture of plant
and animal, like the lichens you have elsewhere. The Disans have a
creature they call a "vaede" that they use for water when traveling.
It has rudimentary powers of motion from its animal part, yet uses
photosynthesis and stores water like a plant. When the Disans drink
from it the thing taps their blood streams for food elements."

"I know," Brion said wryly. "I drank from one. You can see my scars.
I'm beginning to comprehend how the Disans fit into the physical
pattern of their world, and I realize it must have all kinds of
psychological effects on them. Do you think this has any effect on
their social organization?"

"An important one. But maybe I'm making too many suppositions now.
Perhaps your researchers upstairs can tell you better; after all,
this is