Take a look at these web sites:
Flashback Scuba Products: C-Shell
I have seen the shell that Ryan makes and it is cool.
Here are more shots Cousteaus Calypso divers shell integrated dive systems.
A lot was for show (it looks good in camera), but there were many functional features design into the systems.
scuba museum
Cousteau Museum
For the most part shells are often not worth their complication, but they are cool. From the reports that I have heard, some are great to dive with, but not all.
My impression is that a lot has to do with the balanced feel the diver may get out of multiple tanks versus a single large cylinder. At the low speed that we swim, the hydrodynamic drag is not as big of an issue as it is often assumed. Of course there is drag and you want to reduce it, but adding a the fearing probably reduces drag less than
for example putting your arms streamed lined next to your body rather than in the praying position that many divers use now a days.
All that being said
due to the cool factor, at some point I may have to get one of Ryan shells.
Ryan modeled his after the Technisub RAID system (
odd_equipment ), which was a very well design system with many features not available in other dive units.
There are definite advantages to our present modular approach to dive gear, but like in everything, there are some compromises.