2 2 PLATYHELMINTHES TURBELLARIA chap. 



Schizoporella, on the French side of the Channel, and cannot 

 long endure separation from its natural habitat, to which it 

 is adapti very 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 Poly- 

 clinid and other Ascidians, Cycloporus appears part and parcel of 

 the substratum, an interesting parallel to Lamellaria perspicua, 1 

 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, 2 there 

 is only one case, that of Planocera inquilina, 3 in the branchial 

 chamber of the G-asteropod Sycotypus canaliculatus, which would 

 seem to bear the interpretation of parasitism. The jet-black Pseudo- 

 ceros velutinus and the orange Yungia aurantiaca of the Medi- 

 terranean, are large conspicuous forms with no attempt at con- 

 cealment, 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 exist- 

 ence 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. 4 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 Oligo- 

 cladus 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. 



Locomotion. Locomotion is generally performed by Poly- 

 clads at night when in search of food, and two methods, creeping 



1 Cambridge Natural History, vol. iii. p. 74. 



2 Lang, " Polycladen," p. 629. 



3 Wheeler, Journal of Morphology, vol. ix. part 2, 1894, p. 195. 



4 Many Nudibranchiate Mollusca undergo this change of habitat. See Garstang, 

 Journal of the Marine Biological Assoc, n.s. i. ~No. 4, 1890, p. 447. 



