Biggest invertebrate you have ever seen or kept?

[Kapachuka3]

Just curious what is the biggest invertebrate anybody has ever seen or kept. I just saw a purple pincushion urchin at the fish store and it was about the size of a small basketball.

 

[wil-yuhm]

Stepped on many offshore walking waist deep (Horseshoe Crabs):
Water:

Said to run over desert landscapes screaming and hissing under fire (Camel Spiders).
Land: