74 



examined, I found stellate spicula imbedded sparingly in the intersti- 

 tial fleshy matter of the animal, which were strikingly different from 

 those in the corresponding structure of Halichondria Johnstonia ; 

 and in the same sponge I found a very minute variety of stellate spi- 

 cula in the skin or crust, in place of the fusiform ones observed in the 

 crust of H. Johnstonia. The peculiar and characteristic forms of 

 these organs, which appear, within certain limits, to be constant to 

 their separate types, not only in each species, but in each separate 

 part of the same species, strongly indicate the necessity of searching 

 for our specific characters in the minute anatomical characters of the 

 animal, rather than in the vague and changeable characters afforded 

 by external form and general aspect ; and I have but little doubt that 

 when we have sufficiently matured our knowledge of their anatomical 

 structure, many species, and especially that now under consideration, 

 will be removed from their present situations, and become the types 

 of new and well-defined genera. 



The extraordinary structure exhibited in the arenaceous fibres of 

 the skeleton of the two sponges last described, will probably lead to 

 the establishment of an entirely new division in the order ; and a se- 

 cond new division will most probably be hereafter founded on the 

 structure of the new and very curious sponge Dictyochalix pumiceus, 

 described by Mr. Stutchbury of the Bristol Institution, at the last 

 meeting of the Zoological Society. In this sponge the whole of the 

 fibrous skeleton is composed of silex, so transparent, that it has the 

 appearance of being formed of spun glass. These, I repeat, will pro- 

 bably be some of the results arising out of the present and similar in- 

 vestigations of the numerous members of the sponge tribe ; but at the 

 present period our knowledge of the curious and beautiful tissues that 

 are already known, and those that will, without doubt, be hereafter 

 found to exist, is comparatively so slender as to render it expedient to 

 refrain from systematizing, until we attain a more complete knowledge 

 of the details of their anatomy. 



Of these organs the spicula especially require a careful re-exami- 

 nation and classification. Dr. Grant, in his laborious and valuable 

 papers on " The Structure and Functions of the Sponge," in vol. xiv. 

 of the ' Edinburgh Philosophical Journal,' and in vols. i. and ii. of the 

 new series of the same work, has minutely and accurately described a 

 considerable number of forms that occur in British species ; but how- 

 ever numerous the forms may appear that he has there described and 

 figured, they are in reality but few compared with the number that yet 

 remain unfigurcd and undescribcd. 



