AND PHTSIODOGT OF THE SPONGIADiE, 
777 
forms the half of a regular oval figure ; the opposite apices being separated to the extent 
of about the length of one of the radii Each ray is in form like a double-edged blunt- 
pointed knife, bent near the handle in the direction of a line at right angles to one of 
its flat sides ; and each ray is strengthened and connected with the shaft of the spiculum 
by a stout curved web of silex, which extends from a little below the inner surface of 
the ray to a point on the shaft about opposite to its middle. The shaft is cylindrical^ 
and has short stout tubercles dispersed over all its parts when fully developed. 
The structure of every part of this singularly beautiful spiculum is strikingly indica- 
tive of its office in the economy of the sponge ; the form and mode of bending of the 
radii, with their thin edges at right angles to the line of force in a struggling animal, 
and the powerful web at the base of the ray enabling it to sustain an amount of stress 
that the unsupported flat ray would never otherwise be able to endure. 
The spiculated cruciform spicula are exceedingly abundant in every part of the sponge, 
and no victim entangled and retained by the large multihamate spicula could avoid in- 
numerable wounds while struggling to efiect its escape ; while the one held it secure 
within the sponge, the others, from the peculiarity of their form and mode of dispo- 
sition of their acutely pointed spines, would readily release it after the infliction of every 
puncture, only that the wounds might be multiplied until the creature was pierced in 
every part, and bled to death for the nutrition of the sponge. 
Fig. 3, Plate XXXI. represents a small portion of the skeleton of the sponge with the 
two forms of defensive and aggressive spicula in situ, magnified 50 linear. Fig. 4 repre- 
sents one of the multihamate bihamate spicula with a power of 83 linear, displaying the 
adaptation of its structure to purposes of retention. Fig. 5 represents one of the spicu- 
lated cruciform spicula on the same scale as fig. 4, showing their relative proportions, 
and fig. 6 the same form of spiculum with a power of 260 linear, to exhibit the pecu- 
liarities of its spination. 
It would be almost an endless task to describe every variety of these singularly beau- 
tiful contrivances for combined defence and offence in the interior of the Spongiadse. 
Those which I have particularized are some of the most elaborate and beautiful that I have 
seen during the course of my researches. In many other cases, where all that is required 
is defence, the means employed are of a much more simple nature. We find in the 
Spongiadm, as in other animals, that nature frequently economizes her means by the con- 
version of one organ to the purposes of another by slight adaptations or additions ; thus 
\rLHalicliondriaincrustans, Johnston, and in other sponges, the skeleton spicula are made 
to perform the duties of internal defensive spicula, by being more or less furnished with 
spines, as represented in fig. 30, Plate XXIII. Phil. Trans. 1858, and in other cases we 
find them medially or apically spined, as in figs. 32, 33, & 34 of the same Plate. 
In like manner we find the spicula of the sarcode, by the extreme profusion in which 
they occur in that substance near the surface of some sponges, are turned to good 
account for the general purposes of external and internal defence, as well as for their 
special purpose of the protection and support of the sarcode. So likewise in the tension 
