44 INVERTEBRATE ANIMALS. 



inhabits branching cavities in shells, which it excavates for 

 itself. It apparently lives upon the animal matter contained 

 in the shell, and few oyster-shells can be picked up upon our 

 shores which do not exhibit the perforations and mines of 

 some species or other of this genus. Fossil shells, also, often 

 occur, which show that these mining sponges have enjoyed a 

 vast antiquity. 



