Tylodina fungina from California

Dear Chris,
Thanks for these photos. They give us quite an interesting look at some of the external features of this primitive opisthobranch. Firstly it has a large, conical, limpet-shaped shell, and you can see the slightly recurved larval shell [protoconch] right at the tip. The shell is covered by a horny layer [periostracum] - all very snail-like for a slug! Tylodina has a pair of large enrolled head tentacles [rhinophores] like those found in the related pleurobranchs. It differs from its shelled relative, Umbraculum, in having a much more elongate body, and in having a 'snout' extending out from the front of the body. In Umbraculum, the mouth is at the bottom of a groove down the front of the circular body.

One interesting feature we can see in your upper photo, is a light yellow crescent shaped gland, at the front of the mantle, under the shell. This is almost certainly the same organ as the 'pre-mantle gland' reported by Burn (1960) from the Australian Tylodina corticalis. I can't find any reference to it being present in T. fungina.

• Burn, R.F. (1960) On Tylodina corticalis, a rare opisthobranch from south-eastern Australia. Journal of the malacological Society of Australia, 1(4): 64-69.