1869.] DR. J. 8. BOWERBANK ON SILICEO-FIBROUS SPONGES. 349 
liarities of the animal. Thus the true natural positions and mode 
of arrangement of the stout attenuated rectangulated hexradiate spi- 
cula, the full series of the varieties of which are figured in the ‘ Philo- 
sophical Transactions,’ 1858, plate 25. figs. 24-33, and in Mon. Brit. 
Sponges, vol. i. plate 7. figs. 174-183, are well exhibited in situ, 
which I have never yet seen in any of the well-washed specimens 
with which we are now so familiar. In the large lateral orifices of 
such specimens they are sometimes entirely wanting, or a few only 
of them are found in the neighbourhood of the large circular area. 
In the specimens in which they are held in their natural positions by 
the sarcodous and membranous tissues, they are regularly disposed 
around the circular area, forming a compact marginal ring, their 
stout radii projecting in every direction among the surrounding por- 
tions of the skeleton, but not within the circular area; so that where 
one of the radii would, by the natural laws of development, have been 
found, its production is arrested, and it is represented by only a 
slight tumefaction on the axis of the spiculum ; hence it is that we 
find such numerous varieties of form among these remarkable spi- 
cula. All the other radii immersed in the surrounding structures 
are completely developed, crossing each other in every direction ; so 
that although uncomnected by siliceous cementation with the fibrous 
skeleton, they form a strong but somewhat expansile marginal band 
to the circular area. We are thus enabled to perceive the reason of 
the numerous cases of the suppression of frequently several of the 
radii of these marginal spicula, and to read the important fact from 
their positions and modifications that their production is as much 
reculated and modified by the structural necessities of the organs in 
which they form important parts, as are the bones and other organic 
structures of the most highly constituted animals. 
The true positions of the slender rectangulated hexradiate spicula 
with elongated basal axial rays are also well determined in these 
specimens ; they are seen in considerable numbers in the interstitial 
cavities of the sponge, supporting the interstitial membranes, and 
vastly increasing the amount of surface in those vital organs. 
- The trifurcated attenuato-hexradiate and floricomo-hexradiate spi- 
cula are not very numerous; they are irregularly dispersed on the 
sarcodous membranes of the sponge, and are completely immersed in 
the sarcode, and without the aid of Canada balsam are usually invi- 
sible. 
DESCRIPTION OF THE PLATES. 
Puate XXT. 
Fig. 1. A portion of the rigid skeleton of Zphiteon panicea from the specimen 
from Porto-Rico, in the Museum of the Jardin des Plantes, Paris, ex- 
hibiting the confluent structure of the rotulate areas of the skeleton, 
Senet hexradiate spicula, and a few gemmules zm situ, x 108 
inear. 
Fig. 2. A section at right angles to the surface of Zphiteon beatriz, exhibiting 
the confluent rotulate structure of the rigid skeleton, the fusciculi of 
acerate spicula, and the yerticillately spinous retentive spicula i sétw, 
108 linear. 
