108 ANATOMY AND PHYSIOLOGY 



the dermis, as represented by Fig. 310, Plate XX, and 

 Isodictya varians, Bowerbank, is always furnished with a 

 fine network of spicula, the reticulations consisting of a 

 single series of spicula only, and on this framework the 

 dermal membrane is firmly cemented. Fig. 309, Plate XX, 

 represents a small portion of this dermal reticulation, mag- 

 nified 108 linear. 



In Halicondria panicea the same description of reticula- 

 tion prevails, but in this sponge the fibres of the network 

 are always composed of numerous spicula cemented 

 together, as represented in Fig. 303, Plate XIX, illustrating 

 the porous system of the above-named species of sponge. 

 But this regularity of structure is not constant even in the 

 same individual; thus, in Hd. panicea you will often 

 observe one portion of the dermis beautifully reticulated, 

 while a closely adjoining spot will be supported by a series 

 of matted spicula, without any indication of areas for the 

 pores, and these variations in structure are evidently de- 

 termined by the presence or absence of those organs at 

 particular parts of the surface. In other cases, beside a 

 general attachment of the inner surface of the dermal mem- 

 brane to the surface of the skeleton, we find it supported 

 by numerous flat fasciculi of spicula dispersed irregularly on 

 its inner surface, and diflering materially in size and form 

 from those of the skeleton, as in our common British 

 species, Halichondria incrustans, Johnston. Great variety 

 exists in these modes of strengthening and supporting the 

 dermal membrane ; but those which I have described above 

 will suffice to illustrate the general principles of their appli- 

 cation. Beside the general systems of external defence, the 

 dermal membrane is often supplied with special defences. 

 Thus, in Tethea muricata, Bowerbank, MS. (Figs. 304 and 

 305, Plate XIX), we find its outer surface abundantly sup- 

 plied with elongo-stellate spicula, which project externally 

 to a considerable extent; and in Bictyocylindrus stuposus, 

 Bowerbank, beside the numerous defensive spicula pro- 

 jected through the surface, we find the membrane filled 

 with minute sphero-stellate spicula, which would effectually 

 protect it from the assaults of any minute enemies that 



