A CURIOUS DWELLING FOR A HERMIT-CRAB. 149 



choice but from necessity, and that if they should 

 have had the choice of other forms, they would have 

 chosen a form differing more or less from the kind 

 of univalve they now occupy, then we should have 

 found in this case, Tvhere no obstruction appeared 

 to be in the way, that the crab would have made 

 its house of a form different from that of a spire. 

 But though the spiral form has been adhered to, a 

 slight dift'erence will be observed. The univalves 

 usually occux^ied by the hermits have the spire 

 more or less drawn out into a turreted or conical 

 form, whereas in the present instance the turns are 

 practically in the same plane, which, as we shall 

 see, is rather significant, and may help us in our 

 endeavour to answer the two following questions, 

 viz. : 1 st — Have the spiral univalves in which we 

 usually find the Hermit-Crabs, no matter what 

 species, become from long-continued habit a neces- 

 sary form? Or 2nd — Supposing there had been a 

 number of forms to choose from, ^vould the spiral 

 form have been selected because of its being more 

 suitable to the conformation of the creature? You 

 are, of covirse, aware that the Pagurl have been in 

 the habit of living in spiral univalves for a long 

 time past -at least as far back as post-tertiary times, 

 when Scotland was an archipelago, with icebergs 

 floating about the narrow channels and grounding 

 in the shallows, leaving their debris to remain an 

 interesting study to the coming ages. We know 

 that this is true, not only because we find the re- 

 mains of these crabs in the post-tertiary clays, but 

 because we find in the same clays the shells they 

 \v<\\e actually d^velt in, — shells rubbed and polished 

 on one particular side by having been dragged about 

 by the hermits, just as we find the shells rubbed 

 and polished by being dragged about by the hermits 

 of the present day. It is very likely, then, that this 

 habit was acquired at a very early date — probably 

 much earlier than the time we speak of; and it 

 would be no wonder if a habit so long-continued, 



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