I know, I know. Don't let reality get in the way of a good cartoon -- BUT...

[Amritama]

The world may never know? :O

Jokes aside, I think Bloo is just like any other invertebrate that can move. Like Ub3rD4n mentioned above, a hydrostatic skeleton doesn't seem like it'd be too off (if I remember my Biomech class right, different pressures on a fluid-filled cavity and action of surrounding muscles create a variety of motions). But yeah, I think Bloo's anatomy strongly resembles any invertebrate, since we know our favorite blue blob has no spine.

Along those lines, Bloo's body might be a muscular hydrostat (think human tongue, some worms, and elephant trunk). As in, his body is composed of muscles without a skeletal system or fluid-filled cavity. What would be really interesting with the theory is how the system would work in Bloo's case.

That, or the creator felt that was a very minor point, which it is, and thus never thought up of a valid reason. But muscular hydrostat FTW. =P