804 
DE. J. S. BOWEEBANK ON THE ANATOAIY 
able nature. These forms of spicula occur in several distinct genera of sponges, and 
especially in those having a strong kerato-fibrous skeleton. Their usual locality is on 
the fibre of the skeleton, in which their bases are firmly imbedded, and from which they 
are projected at various angles into the canals and cavities of the sponge, and they are 
very rarely seen on the membranes. In HymerajiMa stellifera (Plate XXX. fig. 3, a) 
and clavata^ Bowekbank, MS., both exceedingly thin coating species, they occur in 
great quantity, but only on the basal membrane ; a portion of them being erect, the 
remainder prostrate. But in another sponge, a remarkably curious parasitical species 
of Hymeniaddon, which, having no fibrous skeleton of its own, covers and appropriates a 
small fibrous Fucus, and converts its anastomosing vegetable stalks into an artificial 
skeleton, closely coating each stalk of the plant with its membranous structure, so as to 
cause them at first sight to be readily mistaken for keratose sponge fibre, the whole 
of the membranous structure abounds with attenuato-cylindrical entirely spined defen- 
sive spicula ; but they are all prostrate and intermingled with the skeleton spicula 
of the sponge when not in contact with any part of the fibres of the vegetable, but 
wherever they are in contact with the plant they instinctively, as it were, assume the 
erect position, and the false skeleton is bristling with them to as great an extent as if it 
were truly a keratose fibrous structure. This feature in the habit of the sponge is 
very remarkable, and highly suggestive of a capability of adaptation to circumstances 
that we should scarcely have expected to find. By the two instinctive habits, first, that 
of converting the plant into an artificial skeleton, and then erecting its spinous spicula 
on its fibres, it at once simulates the habits of a kerato-fibrous sponge, and becomes 
capable of the carnivorous habits that I have attributed to those sponges that are so 
strikingly adapted for preying on intruding annelids or other such small creatures. 
In the species above described, Hymeniaddon Cliftoni, Bowerbank, MS., Plate XXX. 
fig. 9, the erection of the spicula on the adopted skeleton is an established habit, and it 
may be said to be instinctive in the species, but I have observed the same fact in sponges 
not habitually parasitical. I have a specimen of Microdona cantosa, Bowerbanr, MS., 
a British species, in my possession in which some small fibres of a tubular zoophyte 
have been accidentally included during its growth, which the sponge has coated with its 
own tissues, and from these adopted columns defensive spicula are projected in a similar 
manner to those of the columnar skeleton of the sponge. In this case we have an 
instinctive adaptation of an extraneous substance in a sponge in which the introduction 
of foreign substances is the exception, and not, as in other tribes of sponges, the rule. 
In Hyalonema mirabilis, Gray, a sponge nearly related to the genus Alcyoncellwn, we 
find another extraordinary series of internal defensive spicula, the structure of which I 
have described at length under the head of ‘ Defensive Organs.’ These elaborately and 
wonderfully formed weapons are evidently destined for other purposes than that of 
simple repulsion. The spiculated cruciform spicula, with their short stout basal radii 
planted firmly on the lines of the skeleton, and projecting from their centre at right 
angles to their own plane, the long spiculated ray furnished with numerous strong sharp 
