Ill PROTECTIVE COLORATION IN NUDIBRANCHS J I 



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 

 jjapillae, 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 ^ 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 

 connection with the liver, and no special function to perform. 

 This is the case with Tritonia, Ancula, and Dcndronotus. 



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 colom"s, 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, Alcyonium digitatum, 

 and nowhere 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 tlie varied colours of the Alcyonium 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, 



1 1!. F. ScliarfT, Sci. Traiis. R. Duhl. Soc. (2) iv. p. 553 f, 

 - Q. Journ. Micr. ScL N. S. xxxi. (1890) p. 41 f. 



