1^2 Dr Grant's Observations on the Structure 



that, in the branched species, the central cavities of the horny 

 , filaments are filled with a soft white matter, and that they ter- 

 minate by distinct apertures on the surface of the body ; and he 

 considered these cavities as undoubtedly the habitations of ani- 

 mals of a particular kind, (Hist, des Cor. p. 94). The confir- 

 mation of this opinion, by accurate experiments, would establish 

 a very striking distinction between these elastic species and the 

 more friable earthy sponges of our own shores, and would point 

 out a remarkable approximation in these highly organised spe- 

 cies to the polypiferous axis of tubularise, sertulariae, and other 

 keratophytes. In all the calcareous sponges which I have hi- 

 therto examined, we invariably find triradiate spicula, which are 

 completely enveloped in the connecting matter, and are employ- 

 ed in forming the bounding fasciculi of the pores. Besides these 

 complicated spicula, we frequently find a second and simpler 

 form of spiculum, one extremity only of which is immersed in 

 the connecting matter, while the other end, projecting free from 

 the surface, defends the entrance of the pores and orifices. 

 Thus, in the S. compressa (Fig. 23.), the bounding triradiate 

 spicula (Fig. 11.), of various sizes, are found enveloped in the 

 tough connecting matter around the pores, the defending clavate 

 spicula (Fig. 12.) have their straight tapering portion immersed 

 in the connecting matter, while their curved extremity hangs 

 free over the entrance of the pores. In the S. coronata the con- 

 necting matter seems to cover entirely the bounding triradiate 

 spicula (Fig. 17.) ; and only the thick obtuse extremity of the 

 needle-shaped defending spiculum (Fig. 18.) is immersed in it, 

 while the tapering pointed end hangs free over the pores and 

 fecal orifice. I have never observed a combination of calcareous 

 and silicious spicula in the same sponge, nor any kind of spicu- 

 lum in the horny species. Two distinct forms of spicula are 

 very seldom observed in silicious sponges, though they are fre- 

 quent in the calcareous species. In the Spongia ventilabrum, 

 Lin., besides the long waved silicious filament (Fig. 5.), we ob- 

 serve a distinct needle-shaped spiculum obtuse at one end, and 

 tapered to a point at the other, (similar to Fig. 18). In the 

 S. pilosa, Mont., besides the long straight fusiform spiculum, 

 we observe a shorter curved spiculum, of equal thickness through- 

 out, and rather obtusely pointed at both ends, like that of the 



