OF THE SPONGIAD.E. 



There is the same design as that exhibited in the construc- 

 tion of the dermal areas in Geodia M'Andrewii and Baretti, 

 but there is a considerable difference in the application of 

 the areas produced by the combinations of the ternate 

 apices. In Geodia these areas are placed beneath the highly 

 organized and regularly formed intermarginal cavities, and 

 form the framework and support of their valvular proximal 

 ends ; while in Dactylocalyx Prattii they are situated above 

 the distal ends of the intermarginal cavities of the sponge, 

 which have not the regular structure and valvular appendage 

 of those of Geodia, but are similar to the like organs in the 

 Halichondroid sponges, and in this position they serve only 

 to support and strengthen the dermal membrane, which 

 adheres firmly to their distal surfaces. In this situation 

 they are subject to a greater chance of pressure and dis- 

 ruption than in the more deeply seated ones of Geodia, and 

 accordingly we find extra provisions for the safety of the 

 junctions of their radii. The shafts of these spicula are 

 short, stout, and conical, and they penetrate but a very short 

 distance into the substance of the sponge. They do not 

 appear to be cemented to any part of the rigid siliceo-fibrous 

 skeleton, but are merely plunged into a somewhat thick 

 stratum of membranous structure reposing on the surface of 

 the skeleton. Their radii are compressed considerably and 

 extended laterally, so that their planes are in accordance 

 with that of the dermal membrane, and they present a 

 greater amount of adhesive surface than those having 

 cylindrical radii. The ternate rays ramify irregularly. 

 Sometimes one ram us, after slightly pullulating, remains 

 nearly obsolete, causing the branch to assume a geniculated 

 form, like some of the ramifications of a deer's horn, and 

 no two appear to be exactly alike ; in fact, there is every 

 appearance that each ray is influenced and modified in its 

 development by the necessities of combination with the 

 adjoining spicula, and their apices are directed in such a 

 manner that they lap over each other in opposing lines, so 

 that each two form a spliced joint, giving a much greater 

 amount of strength than the mere crossing of the radii at 

 various angles, as in those of Geodia. The inhalant areas 



