BRITISH SPONGIAD^. 213 



it separates spontaneously, by contraction, from the sides 

 of the canals which it generally inhabits, and its surface, 

 when examined beneath the microscope, appears perfectly 

 smooth and even. The dermal membrane, in its structure, 

 does not appear to differ in any respect from the interstitial 

 one, both membranes being abundantly supplied with the 

 spicula of the skeleton. 



The form of the base of the spiculum varies considerably 

 in the same individual ; some are regularly oval, others 

 ovate, but the greater number of the adult ones are more 

 or less enormi-spinulate. In some specimens the amount 

 of variation from the normal form is greater than in others ; 

 but in none of them do the varieties exist to so great an 

 extent as to induce us to accept them as the normal form 

 of the sponge spiculum. The young spicula are frequently 

 acuate, and they may be seen passing through every gra- 

 dational development up to the well-produced adult enormi- 

 spinulate form. 



The sponge projects for about a line and half from each 

 of the external terminations of the excavated canals, and 

 each of these projections is furnished with an osculum. 

 These organs have been so ably and accurately described 

 by Dr. Grant, in the ' Edinburgh New PhU. Journal,' vol. i, 

 p. 78, and vol. ii, p. 183, that I cannot do better than 

 quote his description of them. " The projecting tubular 

 papillae possess a complicated structure and a high degree 

 of contractile power, and exhibit a singular series of ap- 

 pearances when the Zoophyte is attentively examined while 

 at rest in pure sea water. When under water, the papillae 

 are seen projecting from the apertures of the shell, some- 

 times to the length of a line and a half; they present a 

 wide, circular opening in their centre, and a rapid current 

 of water issues constantly from them, conveying, occasion- 

 ally, flocculi of a gray, membranous matter. But on being 

 touched with a needle, or withdrawn from the water, the 

 opening gradually closes, the current ceases, and the whole 

 papilla, continuing slowly to contract, is withdrawn com- 

 pletely within the aperture of the shell. The papillae, 

 viewed in their contracted state, present a smooth, rounded. 



