J. G. WALLER ON THE BURROWING SPONGE. 275 



the shape of the sponge itself is defined by them. In this case 

 the organ, which performs this office, was that which first led to 

 the discovery of the sponge by Dr. Grant. Though this eminent 

 observer referred it to a zoophyte, he describes the organ itself with 

 characteristic accuracy. He says " The projecting tubular papillae 

 possess a complicated structure, and a high degree of contractile 

 power, and exhibit a singular series of appearances when the 

 zoophyte is attentively examined whilst at rest in pure sea water." 

 When under water, the papillae are seen projecting from the 

 apertures of the shell sometimes 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 occasionally 

 flocculi of a grey 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 a needle, or withdrawn from the water, the opening gradually closes, 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 con- 

 tract, is withdrawn completely within the aperture of the shell. 

 The papilla? viewed in their contracted state, present a smooth, 

 rounded, short extremity ; but when they begin to advance beyond 

 the surface of the shell, their extremity becomes flat and slightly 

 dilated, assumes a villous appearance, with open fissures radiating 

 from the centre to the margin of the papilla, and at length a minute 

 circular opening is perceived in the centre of the villous surface. 

 The papilla advances from the shell, and its central opening enlarges 

 in proportion to the healthy state of the zoophyte, and the purity 

 and stillness of the water ; its flat, downy, radiated surface, gra- 

 dually diminishes by the widening of the central opening, till only 

 thin margins are left around the orifice, and the current is again 

 seen to play briskly from it." 



This organ, so well described by Dr. Grant, is the osculum of the 

 sponge, and the small perforated orifices of the shell are, for the 

 most part, thus made use of for its protrusion, and the exercise of 

 the excretory functions. It is rarely that these orifices are not so 

 used, but in some instances, instead of the osculum, they are closed 

 up by dermal membrane, fortified in the manner I have previously 

 described. In one of my specimens I am fortunate in having one 

 of these oscula preserved in its protruded state. (Fig. 3.) Its 

 spicula are, for the most part, concealed by thick fibrous membranes, 

 but appear at the apex of the cone, upon which a grain of silex 

 rests, as if to give an argument to those who support the " boring" 



