INTllODUCTION. 



WnEN I first undertook tlic history of the British 

 SpongiadiB I did not contemplate giving figures of 

 each species, believing that form and colour in these 

 protean animals were frequently so much varied in 

 each as to render them of but very little service in the 

 discrimination of species, and I still, after much addi- 

 tional experience, remain of the same opinion as regards 

 these two characters. But, as my knowledge of these 

 .animals became extended, I saw that the discrimination 

 of nearly allied species might be considerably facili- 

 i1;ated by magnified figures of the peculiarities of their 

 organic structures, such as those of the dermal mem- 

 brane, the skeleton, and especially of the forms and 

 proportions of the spicula. Under these circumstances 

 it became advisable, for the convenience of naturalists, 

 to p'ive fio'ures of the most characteristic forms of 

 each species accompanied by those of the spicula and 

 the most striking portions of their organization, when 

 necessary., for the determination of the species. In 

 thus figuring the spicula, an average-sized adult one of 

 each form that occurs on the sponge is given, along 

 with the figure of the species, and where nearly allied 

 ones have the forms of their spicula closely resembling 

 each other, but varying to some extent in size, they 

 are each figured of the same linear power for com- 

 parison. 



When I have had a choice of specimens of any parti- 



