OF THE SPONGIAD.E. 67 



fibrous tissues incorporated with the structure. In the 

 contractile membranes forming the oscular diaphragms in 

 Grantia, and in those at the base of the intermarginal 

 cavities in Geodia and Pachymatisma, they attain a greater 

 degree of thickness, and especially in the two latter genera 

 of sponges. In Alcyoncellum, Quoy et Gaimard, the orga- 

 nization of their tissue is still more complex, and we there 

 find them constructed of repeated layers of membranous 

 structure, abounding in primitive fibrous tissue disposed 

 in parallel lines in each layer, the fibres disposed so closely 

 together as to completely cover the membrane beneath, and 

 the direction of the fibres being at various angles to the 

 axis of the great cloacal appendages of the sponge, so as 

 most effectually to aid in the contraction or expansion 

 of that organ. They are so closely packed together and so 

 intermingled, that I could not ascertain their length but 

 from the gradual 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 con- 

 tinuous stratum, while in 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 are not coincident. 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 

 fasciculi. 



This fibre-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 Stematmuenia. 



Fig. 255, Plate XII, represents a small portion of the 

 lining membrane of one of the great excurrent canals of 



