Circularis muscles

[Graeme]

Hi all, yet again my studies have brought me up against a new term and question, much like an iceberg to the Titanic (OK so maybe not quite as bad as that, but a niggling question nevertheless). Loliginid squid and sepioids have circularis muscles encircling the sucker ring, but what does it actually do!? I couldn't find any info on it. Is it there just to hold the ring in place, like as an extra security measure, or does it allow movement or anything special?

Graeme