174 REPORT ON THE NUDIBRANCHIATE 



science. The Zeit-geist is in many quarters making for a re-examina- 

 tion of the very foundations of the theory of Natural Selection, and I 

 trust that the observations here recorded on the subjects of variation, 

 reproduction, rate of growth, colour, and habits of life may be of 

 some service in furnishing facts as regards the life-conditions of 

 this particular group of animals. I further trust that it may not be 

 difficult for others to separate my own theoretical opinions or con- 

 clusions from the facts themselves. 



During the past year a considerable advance in our knowledge of 

 the value of colour in this group has been made. Darwin^ himself 

 seems to have been very doubtful whether the bright tints of Nudi- 

 branchs, and especially of the Eolididse, really served as a protection, 

 except in a few cases ; but evidence is accumulating rapidly to prove 

 that in this group colour, whether conspicuous or dull, has a very 

 important value for the individual and species. Giard (6) and Herd- 

 man (13) have simultaneously noticed that the colouring of the com- 

 mon Archidoris tuherculata affords a means of rendering it incon- 

 spicuous in the localities and positions in which it is usually found. 

 This species feeds usually upon Halichondria panicea, a common 

 spreading shore-sponge, and the colours of the two are very similar. I 

 may point out that this mimetic resemblance is increased by the posses- 

 sion by the Mollusc of a layer of dermal spicules ; an enemy, knowing 

 by inherited instinct, or far more probably by actual experiment and 

 consequent education, the unpalateableness of the sponge, would be 

 doubly deceived as regards the Nudibranch, both by its resemblance 

 to the sponge in appearance and colour, and also by its similar /eeZ. 

 The case of this Archidoris is, however, somewhat complicated ; 

 individuals are often found with patches of green or violet colour on 

 the back, and these add to the general inconspicuousness by being 

 the tints of overhanging or surrounding weeds. Indeed, these 

 colours are often seen in the sponge itself. Herdman does not 

 appear to have noticed the mimicry between the Nudibranch and 

 sponge, but he has a very interesting case in which the colour of the 

 former perfectly simulated that of the surrounding rock, Nullipores 

 and Spirorhis tubes. 



A case more closely approaching one of true mimicry I came 

 across on the shore at Drake's Island in the early spring of this 

 year. It was that of a small Archidoris Johnstoni fixed on the side 

 of an upright rock between tide-marks, and so exactly resembling a 

 small piece of Halichondria panicea (var. papillaris) that at first I was 

 quite deceived by it. It was situated with the head above and the 

 anus below, the gill-plumes being (as Alder and Hancock state to be 

 usual in the species) half retracted and forming a perforated cone 

 perfectly resembling one of the protruding oscula of the sponge. 

 * Descent of Man, 2ad ed,, pp. 261, 262, 264. 



