STEUCTUKE, ETC., OF CERATA OF NUDIBRANCHS. 55 



appearances which are in some cases protective and in others 

 conspicuous and warning ; and in this, it seems to me, we have 

 an explanation of the extraordinary development and variety of 

 these otherwise mysterious processes of the dorsal body-wall. 



For several years past I have been paying some attention to 

 the colours of Nudibranchs and their variations in connection 

 with their habits and natural surroundings. In October, 1888, 

 I described' a peculiarly coloured Doris (Archidoris) tuber- 

 culata which was especially well protected from observation, 

 and since then I have found the same species repeatedly lying 

 in hollows in the surface of large sponges, generally Hali- 

 chondria pa nice a, and simvilating the colours of its sur- 

 roundings so closely as to be quite inconspicuous.^ Giard has 

 recently noticed this same point on the coast of Normandy, 

 and has also recorded a few other cases of protective colouring 

 amongst common Nudibranchs.'^ 



The view which I have given above in regard to the primary 

 function of the cerata occurred to me early last summer, when 

 observing some of the Nudibranchs in their natural conditions 

 on the shore at PuflEin Island, and I have since brought the 

 theory briefly before the notice of the Liverpool Biological 

 Society and before Section D of the British Association at the 

 recent Newcastle-on-Tyne meeting.^ Since then Mr. Garstang 

 has independently arrived at practically the same conclusions 

 in regard to the function of the cerata from his observa- 

 tion of the colouring and habits of the Nudibranchs at 

 Plymouth. 



I shall now give a few instances from my own observations 

 in support of my views. 



Tritonia (or Candiella) plebeia is fairly abundant at 

 Puffin Island and at Hilbre Island, near Liverpool, and is 

 always found (so far as I have noticed) in these localities 



• 'Proc. Biol. Soc. Liverpool/ vol. iii, p. 13. 



^ I see that Mr. Walter Garstang, in liis recently published " Report upon 

 the Nudibranchs of Plymouth Sound," has noticed this same instance of 

 protective colouring. ' Journ. M. B. A.,' vol, i, No. 2, p. 174. 



^ ' Bulletin Scientifique de la France et de la Belgique,' t. xix, 1888, p. 492. 



* See Abstract in forthcoming volume of Reports. 



