AND PHYSIOLOGY OP THE SPONGIAD^. 
771 
complex. Thus in G. M’Andrewii and G. Barretti the defences are double, one system 
consisting of a continuation of the great radial fasciculi of the skeleton as a protection 
against the assaults of the larger and more powerful assailants, and then of a secondary 
series consisting of an infinite number of minute acerate spicula, based immediately 
beneath the dermal membrane and projecting to a slight extent beyond its external surface, 
effectually protecting it and the porous system of the sponge from the attacks of its 
minute and more insidious enemies. 
Similar modes of external defences exist in various species of Pacliymatisma and 
Bdonemia, but no two species appear to agree precisely in these respects. 
In the genera Microdona and Hymeraphia, Boweebank, MS., differing widely in the 
structure of their skeletons from any of the sponges hitherto described, and frequently 
not exceeding in thickness the substance of a stout sheet of paper or a thin card, the 
same principles of defence are carried out, although their structure is widely different 
from each other. In the first genus, the skeleton of which is formed of short pedestals 
of keratode combined with spicula, each of the pedestals, which reach nearly to the 
surface of the sponge, is terminated with a radiating cluster of long curved and acutely 
pointed spicula, the apices of which pass through the dermal membrane in every direction, 
and thus form a most effectual series of external defences, while their shafts beneath serve 
as the framework of the intermarginal cavities of the sponge (figs. 1 & 2, Plate XXX.). 
In Hymeraphia, where the sponge is less in thickness than the length of one skeleton 
spiculum, and where they pass from the basal membrane of the sponge through the 
dermal membrane, their apices acting as external defensive organs, while their shafts 
form the essential skeleton of the animal, there is an especial provision for their preser- 
vation from injury. Their bases are expanded in the form of large bulbs, so as not only 
to afford a greater surface for attachment, but to allow them at the same time to act on 
the principle of a ball-and-socket joint, giving them a more than usual amount of attach- 
ment, and a power of yielding in every direction to pressure on their apices from without 
(fig. 4, Plate XXX.). The defence of the surface of the Halichondroid sponges is less 
apparent, but equally efficacious; the abundantly spiculous reticulations immediately 
beneath and supporting the dermal membrane, would render attacks of annelids or other 
small predaceous creatures exceedingly unpalatable. 
In the calcareous sponges the spicular defences are exceedingly interesting. In 
Grantia compressa, the distal ends of the great interstitial cells are amply protected by 
numerous fiecto-attenuato-acuate spicula grouped around their porous terminations, with 
their club-shaped ends curving in every direction over them, but in no degree interfering 
with the freedom of their inhalant action. In Grantia dliata they are grouped in circles 
around the distal ends of the interstitial cells, but in this species they are acutely pointed ; 
and when the inhalant system is in a state of repose, they are concentrated at their extreme 
points so as to form an elongate cone, effectually enclosing and protecting the porous 
ends of the cells within them ; but when the inhalant action is in full activity, their 
apices recede from each other until they assume the form of a cylinder, and then freely 
MDCCCLXII. 5 N 
