structure of some Silicious Sponges. S49 



dicularly from the margins to the centre of the pores, in 

 which position they are fixed by a tough and almost horny sub- 

 stance enveloping their bases. Notwithstanding, the regular 

 n etched or jointed appearance on the outer surface of these spi- 

 cula, we perceive with the microscope that they are formed of one 

 piece, and have a distinct continuous cavity within, shut at both 

 ends, like every other cavity observed in marine spicula. They 

 are comparatively strong spicula, from their thickness and short- 

 ness, and are acutely pointed at one end. The connecting mat- 

 ter enveloping their thick ends, resolves itself by maceration into 

 bundles of delicate ligamentous threads ; but this appearance is 

 never to be trusted in sponges wliich have once been allowed to 

 dry, as parts then become hard and fibrous, which we observe in 

 the living state to consist of a soft homogeneous pulp. This 

 third form of spiculum is so very distinct from the two preced- 

 ing, that it is probably not confined to this one species of zoo- 

 phyte, but may be found, like the other two forms, to belong to 

 an extensive series, yet unknown, however, to naturalists, and 

 concealed like the present species in the depths of the southern 

 hemisphere. 



A fourth form of silicious spiculum is seen in the long slen- 

 der asbestine filaments, composing the axis of the Spmig'ia ven- 

 tilahru7n, Linn. When a portion of this fan-shaped sponge of 

 the British, Norwegian and American coasts is kept for some 

 minutes at a white heat by the blowpipe, to remove the animal 

 matter, and is then plunged into strong nitric acid, it be- 

 comes easy to reduce it by the pressure of a glass rod, into its 

 component spicula, which we feel by the pressure as well as by 

 their resisting the acid, to be of a silicious nature. They are 

 neither pointed at their extremities nor notched on the surface, 

 but consist of smooth, long, uniformly thick, transparent, waved 

 rods, obtuse at both ends. Those forming the so-named woody 

 veins of this species, lie close and parallel to each other in dense 

 fasciculi, which are disposed in a longitudinal direction from the 

 base to the apex of the sponge. And the spicula which form 

 the loose porous surface, have one end inserted into the dense 

 central fasciculi of the woody veins, while their opposite end 

 projects outwards at right angles to these fasciculi. The waved 

 direction of the remarkably long silicious filaments of this sponge 



