Naught; Nautilus

The Argo‑naut or Nautilus,
With habits quite adventurous,
A com‑bin‑a‑tion of a snail,
A jelly‑fish and paper sail.
The parts of him that did not jell,
Are packed securely in his shell.
It is not strange that when I sought
To find his double, I found Naught.