OF THE SPONGIAD^. 191 



Suborder II. Spiculo-membranous skeletons. Composed 

 of interstitial membranes, having the skeleton spicula 

 irregularly dispersed on their surfaces. 



The prominent character of this Order is that the spicula 

 of the sponges composing it do not assume either the 

 radiate, fasciculate, or reticulate structural arrangement. 

 The distribution of the spicula on the interstitial mem- 

 branes being without any approximation to order. 



Hymeniacidon, Bowerhanh. 



Skeleton without fibre, spicula without order, imbedded in 

 irregularly disposed membranous structure. 



Type, Hymeniacidon caruncula, Bowerbank. 



In Hymeniacidon the spicula are subordinate to the mem- 

 branous structure, they follow its course and are imbedded 

 without order on its surface. The contrary is the case in 

 Halichondria. The network of spicula in that genus, 

 although irregular, is decidedly the predominant structure, 

 and the membranous tissues are secondary to it, and exist 

 only as interstitial organs. The larger and stouter of the 

 spicula in Hymeniacidon, although dispersed amid the 

 slender ones, may be considered as the representative of 

 the skeleton spicula, while the slender ones are truly those 

 of the membranes, the tension ones. 



In some species the interstitial tissues are constructed 

 diffusely, as in H. caruncula, while in other species, as in 

 H. suherea {Halichondria suberea, Johnston) and a few 

 other closely allied species, they are more than usually 

 compact, so that in the dried state the texture of these 

 sponges are very like that of fine hard cork. From this 

 peculiarity of their appearance in the dried condition, and 

 the exceeding compactness of their structure, I was formerb 

 inclined to believe them to be generically different from the 



