_Helix desertorum_, by its gray-brown colour, harmonises well with the prevailing tint of the desert sands, among which it finds a home. Benson observes that the gaudy _H. haemastoma_, which lives on the trunks of palm-trees in Ceylon, daubs its shell with its excrement. Our own _Buliminus obscurus_, which lives principally on the trunks of smooth-barked trees, daubs its shell with mud, and must often escape the observation of its enemies by its striking resemblance to the little knots on the bark, especially of beech trees, its favourite haunt. Some species of _Microphysa_, from the West Indies, habitually encrust their shells with dirt, and the same peculiarity in _Vitrina_ has already been mentioned. _Ariophanta Dohertyi_ Aldr., a recent discovery from Sumatra, is of a green colour, with a singularly delicate epidermis; it is arboreal in its habits, and is almost invisible amongst the foliage.[155] Many of our own slugs, according to Scharff, are coloured protectively according to their surroundings. A claret-coloured variety of _Arion ater_ occurred to this observer only in pine woods, where it harmonised with the general colouring of the ground and the pine-needles, while young winter forms of the same species choose for hiding-places the yellow fallen leaves, whose colour they closely resemble. _Limax marginatus_ (= _arborum_ Bouch.) haunts tree trunks, and may easily be mistaken for a piece of bark; _Amalia carinata_ lives on and under the ground, and in colour resembles the mould; _Arion intermedius_ feeds almost exclusively on fungi, to which its colour, which is white, gray, or light yellow, tends to approximate it closely; _Geomalacus maculosus_ conceals itself by its striking resemblance to the lichens which grow on the surface of rocks, and actually presumes on this resemblance so much as to expose itself, contrary to the usual custom of its congeners, to the full light of the afternoon sun.[156] Several views have been advanced with regard to the dorsal papillae, or _cerata_, in the Nudibranchs. Professor W. A. Herdman, who has examined a considerable number of our own British species, in which these processes occur, is of opinion[157] that they are of two quite distinct kinds. In the first place, they may contain large offshoots, or _diverticula_, of the liver, and thus be directly concerned in the work of digestion. This is the case with _Aeolis_ and _Doto_. In the second place, they may be simply lobes on the skin, with no connexion with the liver, and no special function to perform. This is the case with _Tritonia_, _Ancula_, and _Dendronotus_.