36 Mr. H. J. Carter on the Anatomy 



boiling a bit of the sponge in nitric acid, or exposing it to a 

 red heat, which will destroy every thing but the siliceous 

 elements of the sponge. 



Muscular Cells. 



In many sponges, especially in the harder and tougher 

 species, chiefly about the dermal layer, there are long fusiform 

 cells, whose central contents are a nucleus and several granules. 

 These cells are often united together longitudinally, in the 

 form of a cord, to form the dermal reticulation, or are massed 

 together so as to form a densely tough, contractile cortical 

 layer. Their shape contrasts strongly with the globular cells 

 in the dermis, as may be seen by my figures ('Annals,' 1872, 

 vol. x. p. 107, pi. vii. figs. 10 & 11) ; while they so closely 

 agree in shape &c. with the fusiform cells of " unstriated 

 muscle," that I have provisionally called them " muscular." 

 I have not been able to make an extended examination of 

 them ; but having often met with them in various sponges and 

 in different parts of the sponge, especially in the Pachytragia, 

 it is to be hoped that some one will give his attention to 

 the subject specially, for their general elucidation, as well 

 as that of many other cells of the sarcode whose specific 

 forms and functions have yet to be particularly described and 

 determined. 



Colour of Sponges. 



The most prevalent colours of sponges are different shades 

 of tawny yellow and brown ; but they may be snow-white 

 or jet-black, golden or bright yellow, scarlet or crimson, 

 green, blue, violet, carmine, and purple, passing into the dark 

 neutral tint of writing-ink — indeed, all the colours of the 

 rainbow. 



Still the prevailing colour of the horny skeleton-fibre is 

 tawny yellow, brown, or grey ; but this is no indication of the 

 original colour of the sponge when invested with its natural 

 sarcode, since in fresh specimens of the officinal sponge the 

 surface most exposed to the light may be black, that less 

 exposed (viz. the sides) purple, and the lower part, which is 

 excluded from the light, almost colourless, or partaking only 

 of the light tawny yellow tint of the interior of the body — a 

 tint derived from the horny skeleton, which, being the only 

 part retained in the officinal sponge, presents the well-known 

 " sponge-colour." 



Thus, in this instance, the colour is confined to the dermal 



