Ill PROTECTIVE COLORATION 73 



when the experiments were conducted under conditions which 

 made the scene as much like ' real life ' as possible, did not 

 agree with Professor Herdman. The Ancula crawled over vari- 

 ous parts of the tank for several days untouched by the fish, 

 who sometimes went close to them and looked at them, but 

 never attempted to taste them. Experiments with species 

 whose colours were protective, such as Dendronotiis, were also 

 conducted, and the decided edibility of these species was estab- 

 lished, the fish competing eagerly for them, and tearing them 

 rapidly to pieces. 



Mr. W. Garstang, of the Plymouth Laboratory of the Marine 

 Bioloofical Association, confirms^ Professor Herdman's views as 

 to the shape and colour of Opisthobranchs. Pleurohranchus 

 memhranaceus is known to secrete, on the surface of the body, 

 an acid which reddens blue litmus paper. It is, therefore, no 

 doubt distasteful to fish, which all abominate the taste of acids, 

 and is conspicuously marked with red-brown and yellowish 

 * warning ' colours. Haminea and Pldline^ on the other hand, 

 are good to eat, and consequently possess ' protective ' colora- 

 tion. Rimcina Hancocki^ which is of a brown colour, crawls 

 over brown mud and weeds, but avoids green weeds, on whose 

 surface it would appear conspicuous. Elysia virldis varies its 

 colour according to its habitat, being green when on green 

 weeds, and dark olive, brown, or reddish brown, on pools among 

 tufts of littoral algae. Green specimens of Hermaea dendritica 

 were kept in captivity, and placed in a dish with green and red 

 sea-weeds. They were never observed crawling upon the red 

 weed, upon which they would have been very conspicuous. 

 Archidoris flammea occurred on bright red sponges, to Avhich 

 its colour was so closely assimilated that Mr. Garstang at first 

 quite overlooked it. Goniodoris castanea was found under 

 stones, feeding on compound Ascidians (^Botryllus)^ which it 

 sufficiently resembled to be very inconspicuous in that posi- 

 tion. 



Again, Jorunna Johnstoni lives ^ upon stones on our southern 

 coast, associated with a certain sponge (^Halichondria sp.')^ which 

 it resembles so closely in outline, in colour, in character of 

 surface, and in its projecting plumes, as to make it very difficult 



1 Journ. 3Iar. Biol. Ass. N. S. i. p. 418 f. 



2 Garstang, Couchologist, ii. p. 49. 



