They had been torn to shreds in their first innings by the bowling of Steve Magoffin, losing eight wickets to the Australian and being dismissed for just 76.
Then, with the removal of Magoffin having seen Sussex's response concluded at 300, Somerset again found themselves in trouble on the batting front. Magoffin took another wicket, while there were three for James Anyon and two for Chris Jordan.
Division One leaders Middlesex moved from 11 for two to 31 for four in a rain-interrupted morning session at Durham as they replied to the hosts' 259 all out.
Joe Denly and Dawid Malan were the men removed, both via the bowling of Graham Onions.