I really like the shell and neck - they're organic yet still a bit geometric in shape, very in keeping with both the original ravana and shivan design ethos, along with a number of other parts. Kudos for those sections.
The girders just completely stick out though - they immediately strike me as way too geometric and regularly shaped to fit in well. Some sort of vaguely geometric skeletal/crab-like form would probably work much better here. As they are they just look out of place.
My biggest gripe is with the pipey/tubey things though. They look like hinged metal rods with spikes on them now - with not even a vague idea of purpose to them the tubing had. :\
Even if you leave the girders as they are, I'd really,
really recommend bringing the piping/tubular look back to those parts. They're a very prominent feature of the ravana, and would go a long way to restoring propper balance of technology and organics the shell and neck have.
Overall though, it's very well modeled.
The only other thing I'd suggest is what Galemp said - to smooth off a couple of jagged poly edges (such as the outlines of the spines and such).
