of some Calcareous Sponges. 167 



of the fecal orifices are surrounded with the projecting extremi- 

 ties of minute shining spicula. To the naked eye their external 

 surface appears even and villous, and on tearing them open,, 

 their internal surface appears more compact, and the termina- 

 tions of the pores are wider. On tearing a portion of this 

 sponge into minute fragments, and examining them under the 

 microscope, we find, in place of the horny tubular fibres of the 

 S. communis, which Mr Ellis has compared to fine filaments of 

 catgut, the whole axis composed of slender, shining, transparent 

 spicula of regular and constant forms. Two forms of spicula 

 are observed in this species, the one is tri-radiate, consisting of 

 three rays of the same form and size, united at one point, and 

 forming equal angles by their union ; the rays are thickest at 

 their point of divergence, and taper slightly to near their free 

 extremities, where they are brought suddenly to a point. The 

 rays of the tri-radiate spiculum are hollow within, shut at their 

 free extremities, and have no superficial openings ; but their in- 

 ternal cavities communicate freely at their point of junction, and 

 form there a small central reservoir. These spicula vary much 

 in size in the same individual, but their general length is about 

 the sixth of a line, from the extremity of one ray to the extre- 

 mity of another; and I have not observed any difference in 

 their magnitude taken from specimens, one of which was ten 

 times the size of the other. The other spiculum of the com- 

 pressa is the clavate, which is broadest and rounded at one end, 

 from which it tapers regularly to a point at the other ; it is 

 quite straight for two-thirds of its length from the pointed end, 

 but the remaining thick part is bent so as to describe the fourth 

 part of a circle. This spiculum is distinctly tubular, and shut 

 at both extremities. The very small straight spicula, which we 

 always observe along with these two, appear to be only broken 

 rays of minute tri-radiate spicula. These spicula consist of car- 

 bonate of lime, and exhibit no trace of phosphate of lime, on 

 employing the usual agents to detect its presence. When we 

 examine with the microscope the arrangement of these spicula 

 in the compressa, we observe two rays of the tri-radiate spicula 

 contribute to form the polygonal pores, while the third ray 

 serves to defend and maintain a space between the pores for the 

 lodgment of the soft parts and ova of this animal ; the curved 



