STELOSPONGIA 165 



appear therefore as very conspicuous, concave, longitudinally extending 

 aconulous zones. In most cases oscula 5-8 millim. wide are observed on the 

 summits of the digitate processes. 



The living sponge is light purplish brown ; the dry skeleton dark chestnut- 

 brown, pretty stiff, and elastic. 



The skeleton consists of simple main fibres, which are joined by simple or 

 slightly branched connecting-fibres. The main fibres extend longitudinally 

 upwards in the digitate branches of the sponge, closely following the inner 

 side of the superficially situated oscular tube; they continually give off 

 branches which extend upward and outward, and appear curved in a plumose 

 manner. The main fibres have a slightly uneven, undulating surface, are 0-16- 

 0-2 millim. thick, and on an average 0-8 millim. apart, except close to the 

 oscular tube, where they stand closer together. They are free from foreign 

 bodies and terminate in the surface with several slender, more or less pointed 

 branches, in a somewhat pitchfork-like manner. The connecting-fibres are 

 0-04 millim. thick, pretty straight, and slightly angularly bent at the rare 

 branching points ; they are mostly transverse and simple, connecting adjacent 

 main fibres. The meshes are mostly square, with pretty sharp corners, and 

 1 millim. wide. 



The inhalant pores lead into irregular, tangential, radially compressed 

 canals, about 0-1 millim. wide, in which no annular strictures are observed. 

 These open out into longitudinally extending canals, 0-6-1 millim. wide, which 

 lie just below the surface, and from the floor of which the inhalants proper 

 arise. The ciliated chambers are spherical, 0-043 millim. wide, and destitute 

 of special efferent canals. The exhalant canals join to form larger longitudinal 

 canals, which extend upward below the floor of the 5-8 millim. wide oscular 

 tubes which occupy the grooves in the skeleton. There is one such superficially 

 situated oscular tube in each digitate process. The longitudinal exhalants, 

 which form a kind of subdermal system below the floor of the oscular tube, open 

 out into it by small constricted apertures. The membrane which covers the 

 oscular tube outsidfe is perforated, and appears very similar to the ordinary 

 sieve-membranes, which are expanded over the entrances to the inhalant 

 system. The pores in this fine membrane are round, on an average 0-1 millim. 

 wide and about as much apart. In the trabeculse between them stout bauds of 

 slender spindle-shaped muscular cells are observed. A terminal osculum may 

 be present or absent. There can be no doubt that the water is expelled by the 

 small pores in the membrane covering the oscular tube, and it appears very 

 remarkable that the structure of this membrane should be so similar to that of 

 a common inhalant pore-sieve. 



Geographical Distribution. — South coast of Australia (Gunn). East 

 coast of Australia : Port Jackson, N. S. W. (Lendenfeld) . 



