Cases of Mimicry, or protective resemblance, when a species otherwise defenceless adopts the outward appearance of a better protected species, are rare among the Mollusca. Karl Semper[150] mentions an interesting case of the mimicry of _Helicarion tigrinus_ by _Xesta Cumingii_, in the Philippines. It appears that all species of _Helicarion_ possess the singular property of shaking off the ‘tail’ or hinder part of the foot, when seized or irritated. Specimens captured by collectors, _Hel. tigrinus_ amongst them, have succeeded in escaping from the hand, and concealing themselves, by a sort of convulsive leap, among the dry leaves on the ground. This power of self-amputation must be of great value to _Helicarion_, not only as enabling it to escape from the clutch of its enemies, but also as tending to discourage them from attempting to capture it at all. Now the genus _Xesta_ is, in anatomy, very far removed from _Helicarion_, and the majority of the species are also, as far as the shell is concerned, equally distinct. _Xesta Cumingii_, however, has, according to Semper, assumed the appearance of a _Helicarion_, the thin shell, the long tail, and the mantle lobes reflected over the shell; but it has not the power of parting with its tail at short notice. It lives associated with _Helicarion_, and so close is the resemblance between them that, until Semper pointed out its true position, it had always been classified as a member of that group. In the same passage Semper draws attention to two other cases of apparent mimicry. The first is another species of _Xesta_ (_mindanaensis_) which closely resembles a species of _Rhysota_ (_Antonii_), a genus not indeed so far removed from _Xesta_ as _Helicarion_, but, as far as the shell is concerned, well distinguished from it. In this case, however, there is no obvious advantage gained by the resemblance, since _Rhysota_ as compared with _Xesta_ is not known to possess any definite point of superiority which it would be worth while to counterfeit. A second case of resemblance between certain species of the genus _Chloraea_ and the characteristic Philippine group _Cochlostyla_ will not hold good as affording evidence of mimicry, for _Chloraea_ is now recognised as a sub-genus of _Cochlostyla_. The Mollusca are not much mimicked by creatures of different organisation. This appears at first sight strange, since it might have been thought that the strong defensive house of a snail was worth imitating. Still it is probably not easy for creatures bilaterally symmetrical to curl themselves up into an elevated spiral for any length of time. One or two instances, however, may be mentioned. The larva of a moth belonging to the Psychidae, and occurring in France, Germany, the Tyrol, and Syria, coils itself up into a sinistral spiral of three whorls, and is aptly named _Psyche helix_, a kindred species from Italy being known as _Ps. planorbis_.