772 
DE. J. S. EOWEKBANK ON THE ANATO ItlY 
admit the incurrent streams of water, but effectually repel the advances of any dangerous 
assailant that may attempt an entrance. The distal termination of the cloaca in this 
species is also abundantly protected by a marginal fringe of long and very acute spicula, 
and is furnished with the same simple but beautiful mechanical contrivances for opening 
and closing in accordance with the necessities of the animal. For a more complete 
description of the anatomy and physiology of this highly interesting species I must refer 
my reader to the ‘ Transactions of the Microscopical Society of London,’ vol. vii.p. 79, pL 5. 
In other species of Grantia the same principles of external defensive action exist, 
but the precise mode is never exactly the same in any two species. 
Internal Defensive Spicula. 
The internal defensive spicula of sponges are exceedingly various in their forms and 
modes of application to their especial purposes; and they seem naturally to resolve 
themselves into three distinct groups ; — 1st, those which are destined simply to repel ; 
2nd, those which wound and lacerate as well as repel ; and 3rd, those which are calcu- 
lated not only to destroy but also to retain intruders. 
The purposes of the first class of spicula are frequently performed by the ordinary 
spicula of the skeleton, which are projected more or less into the cavities immediately 
within the oscula and other spaces requiring such protection ; but when especially formed 
for and appropriated to defensive purposes, they are always free from spines and usually 
terminate acutely ; and they are frequently provided with widely extended basal radii, 
so as to fix them rigidly and firmly in their proper positions, as exemplified in the various 
forms of spiculated triradiate spicula represented by figs. 14, 15, 16, & 17, Plate XXIV., 
Phil. Trans, for 1858. 
The best illustrations of the application of the simple defensive spicula are to be found 
in the cloaca in several species of Grantia.^ as in G. ciliata., Johnston, and G. tessellata 
and enscda, Bowerbank, MS. In all these species this great central cavity is abundantly 
furnished with spiculated triradiate spicula, such as represented by figs. 15 & 16, Plate 
XXI Y. Phil. Trans. 1858, which are so disposed that while the basal radii are firmly 
cemented on the surface of the cloaca, the spicular or defensive rays are projected 
from its surface, not at right angles to its plane, but always at such an inclination 
towards the mouth of the cloaca as to present a combined series of sharp points in the 
best possible position of defence, so that an intruding assailant could scarcely escape 
being seriously wounded by them, while a retiring enemy would pass with impunity over 
their inclined apices. In some species, as in G. tessellata., the defensive ray is natm'ally 
cuiA'ed to the desired angle for defence (fig. 16, Plate XXIV. Phil. Trans. 1858), and it 
is also of such a form as to be readily released fi'om the creature it has wounded, eitlier 
by being attenuato-acuate or ensiform, as in fig. 15, Plate XXIV. Phil. Trans. 1858, 
from G. ensata, and as represented in situ by a small portion of a longitudinal section of 
the cloaca of a specimen of Grantia tessellata in Plate XXX. fig. 5, in which the defen- 
sive radii are all curved in the direction of the mouth of the cloaca. 
