Hmm, let's see, where to begin?
Perhaps the lifefrom is arsenic based, or silicon based?
Perhaps it doesn’t have an internal skeleton? Maybe an external one or a skeleton-like organ system.
Differing method of nerve usage perhaps? Having the nerves being made of steel wire rather than neurons seems like a cool idea.
Maybe it's a anaerobic life form and doesn't need to breathe?
Perhaps it has several organs that we humans do not need? maybe it doesn’t have a single heart, or it has no heart at all and the blood pushes itself along?
All good ideas, although with various degrees of plausibility. The best idea would be to consider the alien and how it would need to have adapted in order to survive on it's homeworld.
edit: Animal biology sounds like a fine idea. There is some werid shit out there in the animal kingdom.
...just don't look in the real life section of exotic alien equipment, okay?
edited 14th Feb '11 10:27:57 PM by doorhandle