AND PHTSIOLOOT OF THE SPONGIAD^. 
773 
In the second division the internal defensive spicula are usnally short and straight, and 
more or less covered with strong conical acutely pointed spines, projected either at right 
angles to the axis of the spiculum, or recurved considerably towards its base ; generally 
speaking the spines are dispersed on all parts of the spiculum without any approach to 
order, as represented in fig. 1^ Plate XXIV. Phil Trans. 1858, while in other cases, as 
in figs. 2 & 3 in the same Plate, they are arranged in verticillate order on all parts of 
the spiculum. In each of these varieties the bases of the spicula are usually profusely 
famished with spines, so as to ensure a strong and somewhat rigid mode of attachment. 
There is undoubtedly a special purpose in every variation of the spination of these 
spicula, and in their presence generally. The short strong form and acute distal termi- 
nation admirably adapt them to encounter the larger description of intruding annelids, 
the most dangerous internal enemies of the Spongiadee ; while the spination of their 
shafts presents a series of minute weapons that would prove equally formidable to those 
intruders that were too minute to be affected by the larger weapons of defence. 
The acuate enthely spined defensive spicula are of very common occurrence in sponges, 
and are by no means confined to particular tribes or genera. As a general rule, when 
the external defences are very full and sufficient, we should not expect to find the internal 
defences abundant, and, on the contrary, when there appears to be a paucity of external 
defences, the internal ones are frequently exceedingly numerous. Thus, in the genus 
JDictyocylindrus, Boweebaxk, MS., where in almost every species the surface of all parts 
of the sponge is bristling with the acute terminations of the radiating external defensive 
spicula, although in most of the species we find acuate entirely spined internal defensive 
ones, yet in many of the species they are so rare as to be by no means readily detected. 
When the skeleton is formed of keratose fibres, we find them dispersed on their surface 
without any approach to order, and projected at every imaginable angle. If the skeleton 
be formed of any of the varieties of spicular reticulations, they are based in a similar 
manner on the principal lines of the reticulated structure, and sometimes, but not very 
frequently, they occur in groups. 
I will not extend this portion of my subject to an unnecessary length by describing every 
mode of their occurrence, but select a few of the most interesting cases as illustrations 
of the general principles of their application. 
Fig. 6, Plate XXX. represents a small portion of the kerato-fibrous skeleton of an 
Australian sponge, with the attenuato-acuate entirely spined internal defensive spicula 
in situ. Fig. 7 represents a few fibres from a kerato-fibrous sponge from the West 
Indies, in which the verticillately spined internal defensive spicula are dispersed over the 
fibres ; and fig. 8 represents the same description of defensive spicula from a West 
Indian kerato-fibrous sponge, having the defensive spicula congregated in bundles. 
Sometimes, but not very frequently, they are found on the interstitial or basal membranes 
of the sponge, and under these cfrcumstances many of them are prostrate in place of 
being erect ; and in one sponge, Eymeniaddm Cliftmi, Boweebank, MS., a singular 
parasitical species from Freemantie, Australia, this prostration appears to be effected by 
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