150 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



even though acquired not from choice but from 

 necessity, should have become a permanent charac- 

 teristic. Reasoning in this v^ay, I was therefore 

 inclined to answer the first question in the affirma- 

 tive—that the spiral dwelling which the crab had 

 formed in the sponge was the result of long-continued 

 habit; but further consideration of the problem led 

 me very much to abandon that idea, and ,to ascribe 

 the spiral form of the crab's dwelling in the sponge 

 rather to the conformation of the body of the hermit 

 than to habit. 



In studying the Crustacea it will be observed that 

 among the decapods, to which the hermits belongs 

 the abdomen is either entirely folded underneath, as 

 in the edible crab, or has a tendency to curve more 

 or less downwards, as in the lobster. Now, if the 

 curve of the abdomen were to be continued for- 

 wards, the result would be a more or less convoluted 

 spire, and the convolutions would be very much in 

 the same plane. Such is the form of spire we find 

 to have been formed in the sponge (fig. 2). It is- 

 therefore likely that the spiral shell is selected, 

 even supposing there had been other forms to. 

 choose from, because it is best suited to the confor- 

 mation of the hermit, and not so much from long- 

 continued habit, or because no other form is avail- 

 able. If I am right in my conjecture, then to my 

 mind there is here another evidence of " design in 

 nature," and that 



•' The Lord is good to all : 

 His tender mercies are over all His works ; " 



— providing for all creatures so as to enable them to 

 fulfil the purpose of their being. 



