Defenses of nudibranchs and their relatives include vacuolated skin with protective spindles, considered in this section, and
CAMOUFLAGE (CRYPSIS),
FAST CRAWLING & SWIMMING,
MUCOUS COATINGS,
CERATAL AUTOTOMY,
NUTRITIONAL CONTENT,
SPICULES,
NEMATOCYSTS,
ACID SECRETIONS,
INK & OPALINE SECRETIONS,
SECONDARY METABOLITES,
ALARM PHEROMONES,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

What protection, if any, do nudibranchs or other opisthobranchs have from being stung by nematocysts of their prey, either during feeding or as they brush against them?  A recent study on several nudibranchs in Italy is germane to this question, especially since some of the genera considered in the study are represented by species on the west coast of North America.  Briefly, the authors describe the presence in aeolids that feed on cnidarians of large ovoid discs, called spindles, in epidermal cells of regions exposed to nematocysts, such as the certata, lips, rhinophores, and gas-exchange surfaces, as well as in the stomach linings (see e-micrograph image on Right of one or two ceratal cells in the epidermis of Flabellina).  In contrast, in sponge- and bryozoan-eating dorids, glandular cells predominate. 

Spindles and vesicle cells are absent, as well, in plant-sucking ascoglossans, Their epithelial surfaces are thin and contain abundant glands for mucus production (see image on Left).

The abundance of spindles in epithelia of aeolids which are most exposed to nematocyst discharge by the prey, and the apparent insulating nature of these cushion-like entities, strongly suggest a protective role. The authors propose a dual defensive strategy in nudibranchs. On the one hand, there is glandular production of mucous coats and aversive secretions (defensive chemicals), as in dorids and, on the other, the presence of structural defensive devices that are like cushions stuffed with inert grains, as in aeolids.  Martin et al. 2007 Mar Biol 150: 807.

NOTE  the spindles occur in large vesicles and appear in scanning e-microscopical view to be biconcave discs about 5µm along the long axis, with thick rims, and filled with a meshwork of polysaccharide filaments.  The vesicles (or “vacuoles” or “balloon-like structures”) and their contents have been described many times since the end of the 19th Century, but in a haphazard way, and the value of the present work is in its comparative approach using representatives from 4 suborders of Nudibranchia and from the Order Sacoglossa