244 THE BIOLOGY OF THE SEA-SHORE 



turns round and inserts its abdomen. The whole sequence 

 of movements is, according to this writer, invariable, and 

 its unintelligent nature is proved by the way in which hermit 

 crabs will explore a small wooden ball or practically any 

 convex surface exactly as they do Gastropod shells. 



The experiments of Drzewina (quoted by Bohn, op. cit.) 

 shed further light on the behaviour of these forms. Hermit 

 crabs deprived of their protection were placed in the presence 

 of Trochus shells the mouths of which had been completely 

 closed with plaster. For a considerable time the crabs 

 endeavoured to obtain an entry, but without success. 

 Gradually, however, they became indifferent towards the 

 shells and a week or so after the beginning of the experiment 

 took no further notice on chancing to come into contact with 

 them, but simply climbed over them or pushed them to one 

 side. When, now, shells of another kind and shape were 

 placed in the aquarium with their openings also plastered 

 up, the crabs, on encountering them, at once set about 

 exploring them actively and systematically, turning them 

 round and continually endeavouring to introduce their 

 forceps into the orifice and pulling away pieces of plaster. 

 From these experiments Drzewina concludes that the 

 behaviour of hermit crabs is modifiable by previous expe- 

 rience, and further that hermit crabs are able, by means of 

 their tactile sensitivity, to discriminate between different 

 forms. In fairness, it must be pointed out that the results 

 of Drzewina's experiments tend to show that the behaviour 

 of hermit crabs is less mechanical than Bohn would, perhaps, 

 lead us to believe. While our own attitude towards these 

 problems is admittedly a sceptical one, we are anxious to 

 avoid doing injustice to facts. 



Masking o! Spider Crabs. — The habit which spider- 

 crabs have of planting out their backs with pieces of weed, 

 hydroid, and so on, has already been referred to from the 

 point of view of its significance in the shore struggle. The 

 habit seems to be most highly perfected in species of Dromia, 

 which make use of the Laminarian seaweed among which 

 they live. Pieces of the weed are cut off with the forceps 



