292 ANIMAL DEFENCES 



power of colour - adjustment. Similar observations have been 

 made in the case of Trout. Many other fishes are well known 

 to vary in hue according to their surroundings, and the present 

 writer remembers seeing a striking case some years ago in a 

 marine form known as the Lumpsucker (Cyclop terus lumpus). 

 This fish is a ground-feeder, and an example had been caught at 

 the floating marine station then existing at Granton, near Edin- 

 burgh. It was of a forbidding black appearance, and to keep it 

 alive for further observation was suspended in the clear sea- water 

 within a sort of drum made of net- work, the top of this being just 

 above the surface. After a short time the fish was pulled up for 

 re-examination, and was then found to be of a bright-green colour, 

 strikingly different from its former sooty hue, and harmonizing 

 well with the sea-water. 



It has been proved for Frog and Chameleon, and is no doubt 

 true for fishes as well, that the colour-changes are brought about 

 by various external agents which affect the nervous system, these 

 including not only light but also changes in temperature, contact 

 with surrounding objects, and variations in the amount of oxygen 

 available for breathing. 



COLOUR-CHANGE IN MOLLUSCS. Some of the Sea-Slugs 

 exemplify variable protective resemblance. This is the case, 

 for example, with Elysia viridis, a little creature closely re- 

 sembling a Land - Slug in form and method of progression. 

 Those specimens found among green sea-weeds are green in 

 colour, while brown weeds of various hue harbour individuals 

 which harmonize with their particular tint. (See as to what 

 has been said on p. 285 regarding Ovulum.) 



COLOUR-CHANGE IN CRUSTACEANS. Many of the smaller 

 Crustaceans which are translucent, or it may be transparent, 

 afford most instructive examples of variable protective resem- 

 blance. The Common Prawn (Palczmon serratus), for example, 

 has long been known to become dark in colour when placed on 

 a dark surface, and colourless when allowed to rest on a white 

 surface, but by far the most remarkable case so far investigated 

 is that of the ^Esop Prawn (Hippolyte variant], a species which 

 is common in low-tide pools and shallow water. This has been 

 the subject of an elaborate research by Keeble and Gamble, who 

 have confirmed and extended the results of several previous 

 workers. It is found that these Prawns harmonize in the most 



