In previous posts on this site, I have introduced you to some of the incredible animals that shelter under the misleadingly unprepossessing name of 'sea slugs'. However, marine mollusc diversity being what it is, it should probably come as no surprise that not all that squashes is slug-y.
The Velutinidae (sometimes referred to in older references as Lamellariidae) are a group of small gastropods that often look rather like sea slugs on the outside, but are misleading it that they actually do have shells. In one subfamily, the Lamellariinae, the mantle has expansive lobes that wrap up around the shell, covering it over. Velutinids are far from being the only molluscs that do this: the glossiness of cowrie shells, for instance, is because live cowries have the shell protected from the elements in this way. Lamellariines differ from cowries, however, in that the mantle lobes are more or less fused to each other and cannot be retracted back. In the other subfamily of Velutinidae, the Velutininae, the shell is not entirely covered by the mantle, as can be seen in the photo below. In both subfamilies, the shell makes it through one or two small loops before broadening out into an abalone-like shape. Lamellariines also differ from velutinines in lacking the marginal teeth of the radula (Beesley et al. 1998).
The velutinids are all specialised predators of ascidians (sea squirts), which they tear into with hard chitinous jaws contained in the buccal mass. The mantle of lamellariines is often coloured to look like the sea squirt they are feeding on, which can make them very difficult to see: to a casual observer, they're just one more squashy blob amongst a whole bunch of squashy blobs. The unfortunate sea squirts are used as nurseries as well as dinner: the female velutinid inserts each egg capsule into a hole that she bites into the sea squirt, with only a narrow neck protruding from its skin through which the velutinid larva hatches.
The velutinids are not close relatives of the classic sea slugs, but closer to gastropods such as cowries and periwinkles. They are regarded by most authors as closest to the Triviidae, a group of cowrie-like gastropods that resemble velutinids in their expanded mantle lobes and preferred diet of ascidians. Velutinids and triviids also resemble each other in having an unusual type of larva called an echinospira. Echinospira larvae appear to have two shells, with a mineralised inner shell that is quite separate from a transparent, glassy outer shell. In the Velutinidae, these two shells even coil differently: the outer shell is planispiral, but the inner shell is helical. When the larva metamorphoses, the outer shell is lost. As a result, earlier authors believed that the inner shell corresponded to the true adult shell, while the outer shell was a novelty unique to echinospirae. In recent times, however, a more popular interpretation is that the two shells each correspond to the inner calcareous layer and the outer periostracum of more usual shells, with their growth having become decoupled. The only other gastropods known to possess an echinospira larva are members of the family Capulidae; whether this larval type indicates that all three families form a single clade remains uncertain.
Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis, pt B. CSIRO Publishing: Melbourne.
Lebour, M. V. 1935. The echinospira larvae (Mollusca) of Plymouth. Proceedings of the Zoological Society of London 105 (1): 163-174, pls 1-6.