16 FAIRY LAND AMONG THE SPONGES. 



The reader may remember that the mouth of the vase 

 is closed with a perforated lid woven of the same glassy 

 substance as its walls ; yet this vase is mostly inhabited, 

 two little crabs being generally found within it. How 

 did they get into it, and what are they doing there 1 

 They seem as great a puzzle as the insects in amber used 

 to be. At first some persons thought that the crabs 

 were the real architects of the Euplectella, and that they 

 had woven the glassy basket as a habitation for them- 

 selves. Crabs, however, are so manifestly incapable of 

 producing siliceous threads that this theory never gained 

 much credence, and was soon abandoned. 



In reality they are simple commensals, a term which 

 has already been explained ; and they are by no means 

 singular in this respect. All naturalists have been long 

 familiar with the fact that certain little crabs (Pinno- 

 theres) inhabit the shell of various molluscs, the pinna 

 being the most usually favoured host. They never 

 injure the living pinna, but will feed greedily on the 

 body of a dead one. So, just as the commensal pea- 

 crabs live in the pinna, while the substance of the shell 

 is deposited round them, so do these little messmates live 

 within the Euplectella basket, remaining there while its 

 glassy meshes are being woven around them. Why 

 either of these Crustacea should choose such remarkable 

 habitations no one knows. 



Of these Euplectella there are many species. To all 

 appearance the animated jelly is exactly the same 

 throughout all the t;ibe, but each has its own form of 



