AND PHTSIOLOaT OF THE SPONGIAD^. 
751 
attenuation of some of their terminations, they would seem not to be continuous for any 
considerable distance. On some of the layers of this compound membrane the fibres were 
disposed in an even and continuous stratum, while on others they were gathered into broad, 
flat, parallel fasciculi. When the compound structure consists of several layers of fibro- 
membranous structure, the disposition of the fibres on the different layers is not coin- 
cident. In some cases they cross each other at right angles, while in others the angle 
does not exceed 45 degrees. The latter mode of arrangement appears to prevail in the 
membranes connecting the great longitudinal fasciculi of spicula, forming to a great 
extent the skeleton of the cloacal appendages of the sponge ; while the arrangement at 
right angles appears also in the tissues immediately surrounding the great skeleton fas- 
ciculi. 
This fibro-membranous tissue abounds in the dermal and interstitial structures of the 
sponges of commerce, but the greatest development of this structure is exhibited in the 
genus Stematumenia. 
Fig. 4, Plate XXVII. represents a small portion of the lining membrane of one of the 
great excurrent canals of the common honeycomb sponge of commerce, in the condition 
in which it came from the sea. The primitive fibrous tissue is seen arranged in a single 
layer in parallel lines at right angles to the long axis of the canal, but partially obscured 
by the stratum of sarcode on the membrane. 
Fig. 3, Plate XXVII. represents a small portion of the dermal membrane of a Stema- 
tumenia, in which the primitive fibres are seen wandering in every direction over the 
surface of the membrane. 
Figs. I & 2 in the same Plate represent portions of a stouter and a more compound 
membranous structure, from the walls of one of the great cloacal projections from 
the surface of Alcymcellum robusta, Boweebank, MS. In this case the membrane is 
strengthened by two or more layers of primitive fibrous structure, the parallel fibres of 
each crossing the others at various angles. 
Fibrous Structures. 
There are two well-characterized classes of fibrous structure. 
1st. Primitive fibrous tissue. 
2nd. The fibres of the skeleton. 
1. Primitive Fibrous Tissue. 
The first of these tissues is exceedingly minute. The fibres are cylindrical in form, 
and are usually of considerable length ; but where they are fully developed, they occur in 
such numbers, and in such a matted condition, that I have been unable to separate an 
unbroken one from the mass. They continue through the whole of their length as 
nearly as possible of the same diameter, and there rarely appears to be any attenuation 
towards their terminations, which are usually obtuse. They are evidently very elastic 
and contractile. When partially separated from their attachments to the membranes, the 
free ends seldom remain straight, and most frequently they curl considerably in dif- 
