ACCOUNT OF BRITISH SPONGES. 101 



brown when dry : its shape is indefinite, but it 

 has the singular property of being attached only 

 (as far as I have been able to ascertain) to old 

 univalve shells, which it entirely invests. It is 

 also remarkable, that few instances occur, where 

 the Hermit-crab ^-has not formed a lodgement in 

 the nucleus shell, and there appears to be a great 

 struggle between the two parasitical intruders, as 

 the sponge is continually endeavouring to fill up 

 the aperture of the shell, while the crab, by its 

 occasional motion in search of prey, frustrates 

 that natural propensity of the sponge. Notwith- 

 standing the efforts of so active and restless an 

 intruder, the gradual and insensible increase of 

 the sponge gains upon the premises of the crab ; 

 it pushes in on all sides, and completely lines the 

 interior surface of the shell, so that the crab soon 

 finds its habitation too small, and is compelled to 

 search for a more capacious house. 



The species of shell may frequently be traced, 

 which constitute the nucleus of this sponge. 

 Buccinium reticulatum and undatum, Turbo tere^ 

 hra and littoreus, are frequently covered by this 

 sponge, except more or less opening to the aper- 

 ture of the shell. 



It is a curious circumstance, that no instance 

 has occurred where this sponge has been attached 

 to a living shell, or such as were inhabited by its 

 proper animal. Either the increase of the sponge 

 must be very rapid, or its efforts to inclose the 

 aperture of a shell, must be most successfully per^ 



G 3 



