Oh boy, now I know why you don't see too many animated octopus; these eight armed molluscs are quite a handful to work with! Ok, the idea of setting up this guy was to try and make his tentacles easy to control and as smooth curving as possible. The original plan was to use a technique where a large number of little tentacle bones are constrained to a spline path at distributed ease percentages; then, a more manageable number of control bones are used to bend the control spline easily and smoothly... Well after some testing I deviated from that technique, why you ask, well unless you want to use muscle based animation to adjust the control spline paths I found the magnitude of the spline segments needed constant adjustment as they bent - otherwise the tentacle curves are not smooth. So, on with a different example...
Setup a hierarchy of small bones that align with each segment of the tentacle spline mesh. Assign each mesh segment with its bone, as shown below.
For every two bones, add another bone in a 2nd hierarchy, plus one extra. You'll find it easier to offset the bones while you work, but once you've constrained them, you can always use the translate manipulator to put them within the tentacle mesh.
A 3rd and final hierarchy is added; again, one bone for every two in the 2nd hierarchy. These will be used as the actual animation controls. Check in your Project Workspace and make sure that there are indeed 3 separate root level hierarchies (ie. each is not a parent or child of the other hierarchies).
The next step can be a bit tedious if you have an eight armed octopus to do, but should only take a few minuets per tentacle once you've got the hang of it. Each bone in the 1st hierarchy is constrained with Orient Like constraints to bones in the 2nd hierarchy, and each bone in the 2nd hierarchy is constrained with Orient Like constraints to bones in the 3rd hierarchy. This pattern of Orient Like is what makes the tentacle bend smoothly; any rotation of the 3rd hierarchy control bones will be smoothly interpolated down to the bones that attach to the mesh.
Both the 1st and 2nd bone hierarchies should be hidden to make the character easier to work with. For one happy octopus, repeat eight times ;o)
There are a multitude of different constraint patterns that produce similar smooth bending effects; I've found this one was the simplest to configure, no fiddling with constraint percentages or offsets, it's all at 100%. One drawback to be aware of is that the main control bone hierarchy is not directly aligned with the tentacle mesh. This makes it more difficult to constrain a tentacle arm to other objects in your scene, especially if you want to use IK (ie. you want the octopus to hang onto a rock while being buffeted by a strong underwater current). I'll leave that for another day ;o)