106 ANATOMY AND PHYSIOLOGY 



sponges, although the cilia may be readily seen in vivid 

 action within the open oscula, as I have described at length 

 in my paper " On the Ciliaiy Action of the Spongiadse," 

 published, in the ' Transactions of the Microscopical Society 

 of London,' vol. iii, p. 137, not the sHghtest trace of cilia 

 exists without those organs ; and this seems to indicate that 

 the aerating functions were strictly confined in these 

 sponges to the large intermarginal cavities. 



The same mode of reasoning applies equally well to the 

 intermarginal cavities of Geodia and Pachymatisma, to 

 which it is probable that the cilia are in like manner con- 

 fined. The great valves at the proximal ends of these 

 cavities in this tribe of sponges appear to strongly indicate 

 a decided separation of the functions of aeration and di- 

 gestion ; and if this conclusion be true in regard to the in- 

 ter-marginal cavities of Geodia and Pachymatisma, it will 

 probably be so in the homologous organs in Grantia; and 

 in this case we must look for the digestive surface in the 

 shallow cavities intervening between the terminal valve of 

 the intermarginal cavities and the parietes of the great 

 cloaca, and of the surfaces of that organ itself. The 

 structure and functions of the intermarginal cavities, and 

 especially as displayed in Geodia and Pachymatisma, indi- 

 cates a closer alliance with the great class Zoophyta than 

 has hitherto been suspected to exist. In the one case we 

 have an accumulation of individual animals conjoined in 

 one mass ; in the other, a similar congregation of organs in 

 place of individuals. 



DERMAL MEMBRANE. 



The dermal membrane envelopes the sponge entirely. 

 When denuded of sarcode by partial decomposition, it has 

 the appearance of a simple, pellucid, unorganized membrane. 

 In the living state its inner surface is somewhat thickly 

 coated with sarcode, and it has the appearance of, com- 

 paratively speaking, a stout, tough skin, and in many 

 sponges it requires a considerable -amount of violence to 



