774 
DE. J. S. BOWEEBANK ON THE ANATOMY 
an especial law. This singular sponge envelopes several fan-shaped portions of a Fucus^ 
and systematically appropriates the minute ramifications of its stem to the purposes of 
an artificial skeleton ; the whole sponge abounds with short stout attenuato-cylindrical 
entirely spined internal defensive spicula: but the remarkable circumstance attendant on 
their presence is, that wherever the membranes supporting them envelope and firmly 
embrace a portion of the vegetable stem, they assume an erect position, and exhibit all 
the usual characters of defensive spicula ; but where the membranes merely fill up the 
areas of the vegetable network, they are nearly all of them perfectly prostrate, and appa- 
rently performing the office of tension, rather than of internal defensive spicula. Their 
form also is singular, being attenuato-cylindrical, not having the acute termination that 
is usual in this description of spicula. 
Fig. 9, Plate XXX. represents a small portion of the fibrous stem of the Fucus coated 
by the membranes of the sponge, and covered with spicula ; those immediately over 
the stem being erect, while those on the membrane are prostrate, (a) represents one of 
this new form of internal defensive spiculum X 175 linear. 
In HymerapMa stellifera, Bowerbank, MS., an exceedingly thin coating British sponge, 
the internal defensive spicula present a singular variation from the normal form. In 
this case they assume the shape of an ordinary Florence oil flask, with a somewhat 
elongate neck, and having a beautiful star-shaped apex in place of a stopper. They 
occur in considerable quantities ; their large bulbous bases are firmly attached to the 
strong basal membrane of the sponge, and they are projected thence at every possible 
angle upward into the interstitial spaces. Their apices are crowded with stout acutely 
conical spines, which radiate in all directions. Fig. 3 <z, Plate XXX. represents a group 
of these spicula in situ, elevated by a grain of sand beneath the basal membrane ; and 
fig. 4 5, Plate XXX., one of the same form of spiculum, magnified 260 linear. In this 
form of spiculum, as in that of Hymeniacidon Cliftoni, their purpose seems to be the 
infliction of laceration, rather than that of destruction, by deep wounds. In another 
species of Hymerajyhia, H. clavata, these spicula have the same large bulbous bases as 
those of H. stellifera, but their apices are acute, like those of the normal forms of such 
spicula. In all these cases we observe in their attachments the same approximation to 
the structure of the ball-and-socket joints of the higher tribes of animals, rendering 
them capable of yielding in every possible direction to the struggles of any enemy with 
which they may be entangled. 
In the third division of the internal defensive spicula there is an especial construction 
for retention as well as for destruction. Their apices are usually more or less hamate, 
as represented in figs. 7, 13, & 12, Plate XXIV. Phil. Trans. 1858, and their attach- 
ments to the sponge are usually such as to allow of a considerable amount of flexibility 
or motion. 
I will not attempt to describe the whole of the numerous variations in the modes of 
their application to defensive purposes, but select a few of the most interesting cases as 
illustrations of the general principles of combined internal defence and aggression. 
