Sea-slugs 269 



tills may be the case, as with one kind which lives on 

 tlie green leaves of the alg?e and is itself bright green. 

 But many brightly coloured, white, or otherwise con- 

 spicuous species, do not seek concealment ; whilst 

 again some equally conspicuous species, as well as 

 other dull-coloured kinds, live under stones and in 

 dark recesses. So that with these nudibranch 

 mollusks colour does not stand in any close relation 

 to the nature of the places which they inhabit." 



Since the reawakening of interest in marine 

 biological investigations, Professor Herdman, Mr. W. 

 Garstang, and others, have published observations on 

 the group which prove that form and colour have a 

 most important bearing upon the habits of Sea-slugs. 

 In the case of deep-sea forms observation under 

 natural conditions is impossible, and even of those 

 species that may be found along the shore at low 

 water much remains to be recorded. 



A clue to the probable origin of the Sea-slugs may 

 be found in certain species possessing shells, but 

 which make a point of constructing them of very 

 thin material and covering them with the side flaps 

 from the foot already mentioned. Starting from 

 Actcuon — which has a spiral shell partially covered 

 in front by lobe-like tentacles from the head — to 

 Pleurohranchus or Ai^lysia, whose shell is reduced 

 to a mere flat plate and completely hidden, it is not 

 difficult to understand how with a tendency to 

 increase the covering of the shell, the shell itself 

 would probably be reduced both in size and thickness 

 as the mantle or the foot-lobes, as the case may be, 

 more and more effectually protected the breathing 

 organs. Some species have a spade-like disk, which 



