444 THE OPISTHOBEANOHIATE 



of the sponges upon which it usually feeds, and as these sponges 

 are themselves very variable in colour, a number of differently 

 coloured specimens of this Nudibranch were kept in one of the 

 small tanks in the Laboratory, and fed under similar conditions upon 

 the same pieces of Halichondria, which were obtained of as uniform 

 a colour as possible. After several months no change was detected in 

 the colouration of the Nudibranchs. The species cannot therefore be 

 regarded as possessing the power of variable protective resemblance.^ 



My friend Mr. Rupert Yallentin has several times sent me large 

 individuals of this species from Falmouth, which have been of a much 

 paler colour than is usual at Plymouth, although such individuals 

 occur. 



With regard to the spawning period of this mollusc, I may add to 

 my previous account that specimens which had been living in the 

 aquarium for some time last winter were found to have deposited 

 spawn early in January. 



38. Akchidoris plammea, A. and H. 



Near the Duke Rock a sponge of Desmacidon-like appearance, 

 with prominent oscula, but of a bright red colour, is very common ; 

 and while looking over a quantity of the sponge eai'ly in April last 

 I found an individual of this species feeding upon it, in dimensions 

 just under three quarters of an inch long and half an inch broad. 

 The colour of the Nudibranch closely approached that of the sponge, 

 but had a more orange tinge. There were a few scattered purplish 

 spots in the middle of the back. The animal was very flattened in 

 form and very changeable in shape. Rhinophoral fossae very wide 

 and capacious, and tuberculated at their edges. Rhinophores and 

 branchiae completely retractile. It was occasionally seen to float 

 inverted at the surface of the water. 



On a piece of the sponge preserved in alcohol for identification 

 I subsequently found another, rather smaller specimen of the same 

 species. It was in a conspicuous position on the sponge, and I must 

 have overlooked it when alive owing to the similarity of its colour 

 to that of the sponge. 



If this species should be found to feed generally upon red sponges, 

 the adaptation will be of considerable interest. Perhaps its rarity 

 may be due, as in many other cases, to an insufiicient knowledge of 

 its peculiar habits. I am inclined to believe that the red Dorises 



1 Mr. Poulton (The Colours of Animals, 1890, p. 108) has mentioned the probable 

 existence in this species of the power of adjustment of its colour to that of its surround- 

 ings. Prof. Stewart's specimens, however, were in all probability not tuberculata, but a. 

 distinct species. 



