Ill PROTECTIVE COLORATION IN NUDIBRANCHS /I 



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.^ 



Several views have been advanced with regard to the dorsal 

 papillae, or cerata^ in the Nudibranchs. Professor W. A. Herd- 

 man, who has examined a considerable number of our own Brit- 

 ish species, in which these processes occur, is of opinion ^ 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. 



Professor Herdman is of opinion that although the cerata 

 may in all cases aid in respiration to a certain extent, yet that 

 extent is so small as to be left out of consideration altogether. 

 He regards the cerata in both the two classes mentioned above 

 as " of primary importance in giving to the animals, by their 

 varied shapes and colours, appearances which are in some cases 

 protective, and in others conspicuous and warning." 



Thus, for instance, Tritonia pleheia^ which is fairly abundant 

 at Puffin and Hilbre Is., appears always to be found creeping on 

 the colonies of a particular polyp, Aleyonium digitatum^ and no- 

 where else. The specimens in each colony of the polyp differ 

 noticeably both in the matter of colour, and of size, and of varied 

 degrees of expansion. The Tritonia differs also, being marked 

 in varied tints of yellow, brown, blue, gray, black, and opaque 

 white, in such a way as to harmonise with the varied colours of 

 the Aleyonium upon which it lives. The cerata on the back 

 of the Tritonia contribute to this general resemblance. They 

 are placed just at the right distance apart, and are just the right 

 size and colour, to resemble the crown of tentacles on the half- 

 expanded polyp. 



Similarly, Doto coronata, which, when examined by itself, is 

 a very conspicuous animal, with showy, bright-coloured cerata, 

 is found by Professor Herdman to haunt no other situations but 



1 R. F. Scharff, Set Tram. R. DuU. Soc. (2) iv. p. 553 f. 



2 q. Journ. Micr. Sci. N. S. xxxi. (1890) p. 41 f. 



