_Leptoplana tremellaris_ is frequently associated with colonies of _Botryllus_, and if separated soon perishes, whereas the free-living individuals are distinctly hardy (Hallez). A closely allied but possibly distinct form lives upon the surface of the Polyzoon {22}_Schizoporella_, on the French side of the Channel, and cannot long endure separation from its natural habitat, to which it is adaptively coloured. A striking case of protective mimicry is exhibited by _Cycloporus papillosus_, on the British coasts. This species, eminently variable in colour and in the presence or absence of dorsal papillae, is usually a quarter of an inch in length and of a firm consistency. Fixed by its sucker to Polyclinid and other Ascidians, _Cycloporus_ appears part and parcel of the substratum, an interesting parallel to _Lamellaria perspicua_,[29] though we are not justified in calling the Polyclad parasitic. Indeed, though a few cases of association between Polyclads and large Gasteropods, Holothurians, and Echinids are known,[30] there is only one case, that of _Planocera inquilina_,[31] in the branchial chamber of the Gasteropod _Sycotypus canaliculatus_, which would seem to bear the interpretation of parasitism. The jet-black _Pseudoceros velutinus_ and the orange _Yungia aurantiaca_ of the Mediterranean, are large conspicuous forms with no attempt at concealment, but their taste, which is not known, may protect them. Other habits, curiously analogous with devices employed by Nudibranch Mollusca (compare _Thysanozoon brocchii_ with _Aeolis papillosa_), emphasise the conclusion that the struggle for existence in the littoral zone has adapted almost each Polyclad to its particular habitat. As regards the vertical distribution of this group on the British coasts, _Leptoplana tremellaris_ has an extensive range, and appears to come from deeper to shallower water to breed.[32] In the upper part of the Laminarian zone, _Cycloporus papillosus_, and, among brown weeds, _Stylochoplana maculata_ are found. At and below lowest water-mark _Prostheceraeus vittatus_, _P. argus_, and _Eurylepta cornuta_ occur. _Stylostomum variabile_ and _Oligocladus sanguinolentus_, though occasionally found between tide-marks, especially in the Channel Islands, are characteristic, along with _Leptoplana droebachensis_ and _L. fallax_, of dredge material from 10 to 20 fathoms.