186 BRITISH SPONGES: 



ternal action in the sponge. It is, however, to be observed that 

 the gravel is very intimately mixed up with the membranous and 

 fluid basis, and appears to be essential to the maintenance of the 

 consistency and figure of the species. 



Mr Bowerbank introduces his description of two species of 

 this genus with the following remarks : — " The sponges which 

 I have now to describe are exceedingly anomalous in their struc- 

 ture, especially the first of the two which I received with many 

 others from my friend, Rupert Kirk, Esq. of Sydney, Australia, 

 who procured it from the coast in his immediate neighbour- 

 hood. • 



" Hitherto we have found that where silex has been the mate- 

 rial with which the skeleton of the sponge tribe has been 

 strengthened and supported, that earth has always been pre- 

 sented to us as a true animal secretion ; either in the shape of spi- 

 cula of various forms, or of very minute spherical molecules ; but 

 in this species, although in great abundance, it is decidedly of 

 extraneous origin, consisting of numerous grains of sand built 

 into the substance of the fibrous skeleton of the animal, without 

 any definite arrangement ; the whole being surrounded exter- 

 nally by a coating of the cartilaginous or horny matter of which 

 the foimdation of the skeleton is constructed, and with which we 

 are so familiar in the sponges of commerce. 



" We are acquainted with many instances in which extraneous 

 substances are employed externally to assist as a covering or de- 

 fence ; as among the MoUusca by Trochus agglutinans, which 

 attaches to the exterior surface of its shell the small shells and 

 fragments of bivalves, or of any other substance that suits its 

 purpose. We observe also among the aquatic larvae, that the 

 cases in which they pass so great a portion of their existence is 

 in many species most ingeniously and beautifully built up of 

 fragments and small stems of plants, bits of shells, and other 

 materials ; and in Sabella Belgica, so common on our own coast. 



