768 
DE. J. S. BOWEEBANK ON THE ANATOMY 
forming the bases of the intermarginal cavities. These offices of the ternate spicula are 
not demonstrated in an equal degree of perfection in all sponges in which they occur. 
"Where the organs which they subserve are best and most abundantly developed, these 
forms of spicula are found in the greatest quantities, and in the most regular and perfect 
mode of arrangement, but where the intermarginal cavities or porous areas are in a less 
regularly developed state, they are deficient in a corresponding degree ; thus e'vincing the 
design and purpose of their structure and presence. The most perfect and beautiful 
illustration of their physiological purpose, in their first mode of application, is afforded 
by the dermal membrane of Dactylocalyx Frattii. Here we find their radii overlapping 
each other longitudinally, and cemented together by keratode, forming a continuous and 
regular network, upon the upper surface of which the dermal membrane reposes, and to 
Avhich it is firmly united. The mode in which the radii are united, and the material 
with which they are cemented together indicate a unity of firmness and elasticity in 
the living state that is truly admirable ; and this mode of structure we perceive is 
especially necessary to the action of the dermal membrane, as the whole of the skeleton 
beneath is perfectly rigid and inelastic. Thus while their shafts are deeply plunged in, and 
firmly secured to, the immoveable mass beneath, their ternate apices are capable of such 
an amount of oscillating motion as would be required for the organic expansion and 
contraction of the membranous structure they support. By the action thus generated 
each pair of the united radii would glide in a longitudinal direction upon each other, and 
thus, although in each separate instance the amount of motion would appear to be ex- 
ceedingly small, the aggregate of the whole would afford a very considerable range of 
expansion, as exhibited in fig. 8, Plate XXIX. 
In their second mode of application, that is to the bases of the intermarginal caHties, 
it appears that as their office is different, so their form and the mode in which the radii 
of their apices is connected are also different. Thus at the inner surfaces of the thick 
dermal crust of Geodia IFAndreioii and Barretti, we find them forming a network 
equally regular and continuous as that in Dactylocalyx Frattii, but the mode of its con- 
struction is varied. The radii do not in these cases glide upon each other longitudinally, 
but they cross each other at various angles; and as the whole mass of these sponges is 
fleshy and very elastic, so by this mode of interlacement of the radii a very considerably 
greater amount of expansion and contraction of the reticulated structure is proHded for, 
while at the same time the power of maintaining the common plane of the reticulated 
tissue is equally as great as in the similar structure in Dactylocalyx Frattii. Thus far 
we can trace the physiological purpose of their structure ; but why in one species we find 
their terminations simple as in Geodia IFAndrewii, and furcated as in Geodia Bam'etti, 
or still further complicated as in the dichotomo-patento-ternate form, is a question which 
cannot be so readily solved without a further acquamtance with the species of Geodia 
bearing these forms in a living state. 
The connecting spicula are not always an essential portion of the skeleton, and they 
exist only in comparatively a few genera of the Spongiadse. 
