540 Dr Grant’s Observations on the Structure 
matter, like other horny substances, and no spiculum is obser- 
ved in the dissolved matter or precipitated to the bottom. Its 
fibres, and every thing, of this nature, are best examined through 
the microscope, when they are suspended in water, and viewed 
by transmitted light. In this manner we observe them to be re- 
gularly cylindrical, translucent, of a brownish yellow colour, 
smooth on their external surface, all nearly of the same diame- 
ter, and distinctly tubular ; they are tough, flexible, very elastic, 
generally quite straight, and they anastomose freely and com- 
pletely with each other, through the whole body of the animal. 
Their diameter is about a third of that of a human hair, their 
length between their points of union^varies from a tenth of a line 
to a line, and their internal tubular cavity occupies about half of 
their diameter, so that these horny fibres have a close resem- 
blance to the spicula of many other sponges. From the clear- 
ness of the light transmitted through their central part;, their 
internal cavity appears to be empty, which is not the case in the 
S.Jitlva and jistulosa. They unite at all angles, and they are a 
little dilated at their points of union ; their internal cavities open 
freely into each other, and a small angular reservoir is formed 
at the place where they meet ; they have no intervening connect- 
ing matter, no line of separation can be discovered at the angles 
where they pass into each other, and no opening is perceptible 
leading from their surface into their internal cavities ; so that 
there is a continuous shut cavity in the interior of the fibres 
throughout the body of the largest common sponge, and these 
horny tubes winding round the pores and canals, cannot, there- 
fore, be the cells of any kind of polypi, destined to create cur- 
rents or other motions within the canals of this animal. The 
fibres unite so as to form polygons, whose sides lie almost al- 
ways in different planes. The great elasticity of the: axis shews 
that the orifices and canals, so obvious in this species, could 
not have been formed and left permanent, by any marine worms 
or insects merely traversing its texture ; but must have formed a 
part of its original structure. The internal cavity of the strong 
horny fibres of the S. jistulosa and S. fulva , is completely filled 
with a dark granular opaque matter, which is continued from 
one fibre into another. This opaque matter renders the limits of 
the tubular cavity very distinct, and probably is the cause of 
these fibres being so remarkably hard and brittle, compared with 
