248 THE ZOOLOGIST. 



tank, shows how deeply rooted is this instinctive attempt to 

 escape an enemy by creeping close to some surface, which would 

 normally be the rock it so much resembles in colour. After 

 eight or nine months in an aquarium the movements of this 

 Spider-Crab are as slow and cautious as when the animal was 

 first introduced, and it never seems to acquire the tameness and 

 confidence so often shown by the Shore-Crab in captivity. 



Owing to its olive brown, or sometimes reddish brown, colour, 

 and the irregular surface of its carapace, the Four-horned Spider- 

 Crab is difficult to detect, even when it is standing fully exposed 

 on the brownish rocks of the aquarium. On two or three 

 occasions I have been quite unable to find a certain individual 

 amongst the rocks, even in the small aquarium whose dimen- 

 sions are given above, and, concluding that it had escaped from 

 the tank during the night, have afterwards found it clinging to 

 the under surface of a rock at which I must have looked in my 

 first survey. It is interesting to note that Aurivillius mentions 

 a somewhat similar experience with Hyas.* I wished to find out 

 whether the Spider- Crabs would voluntarily expose themselves 

 on light-coloured sand, upon which their brown bodies would 

 stand out conspicuously, or whether they would avoid it and 

 remain on the rocks with which their colour harmonised. An 

 experiment was therefore made with two individuals which had 

 lived for several months in a tank, the back of which was 

 covered with dull brown rocks and the floor with brown pebbles. 

 The Spider-Crabs were removed to a tank whose back was 

 covered with similar dull brown rocks, and the floor with 

 exceptionally light-coloured sand. It was found that in the 

 second tank the Spider-Crabs, although they exhibited, as in 

 the first tank, a decided preference for sitting amongst the 

 rocks, were nevertheless quite as frequently to be seen on the 

 sand as they had been on the pebbles ; and there was no marked 

 avoiding of the sand such as I had certainly expected to find. 



It is well known that the different species of Spider-Crabs 

 are in the habit of attaching to their bodies foreign materials 

 which act as disguises. The materials which have been used in 



* C. W. S. Aurivillius, " Die Maskirung der Oxyrrhynchen Dekapoden," 

 ' Kongliga svenska Vetenskaps Akademiens Handlingar,' 1 Haf'tet, No. 4, 

 Band xxiii, 1889-90, p, 11. 



