What are the pros/cons of investing in collective trusts in my 401K?

I noticed that some of the authorized investment choices in my 401K portfolio have no ticker symbols. This led me to discover the term "collective trusts." Is there anything special I should know about these? Are they generally advantageous for employees or can they be wiped out if a company gets torpedoed? Here are a few examples:

• PYRAMIS SMALL/MID CAP CORE II
• BLACKROCK US FDMTL LRG CL T
• BLACKROCK EQUITY DIVIDEND CF
• MFS INTERNATIONAL GROWTH

I notice they all have this disclaimer:

"This investment option is not a mutual fund, registered under the Investment Company Act of 1940. A prospectus is not available and shares are not publicly traded or listed on exchanges."

And an optional question for any historians (and out of sheer morbid curiosity), are these internal investment funds similar to the types of entities set up by Enron (Raptor, JEDI, LJM, Chewco, etc)?