and Physiology of the Spongida. 27 



Pores. 



The pores are situated, as before stated, in the sarcode cover- 

 ing the interstices of the dermis, which sarcode is not a homo- 

 geneous substance, but composed of a number of polymorphic 

 nucleated cells or bodies of a particular kind ('Annals,' 1857, 

 vol. xx. p. 24, pi. i. figs. 6 & 7 ; ib. 1874, vol. xiv. p. 336). 

 These cells, together with the intercellular sarcode which unites 

 them into a common membranous expansion, have the power 

 of separating from each other, so as to extemporize circular 

 holes or pores, and close them wherever and whenever it may 

 be requisite. The average size of a pore is about 1 -100th inch 

 in diameter. 



It may open inwardly into a minute canal or into a 

 " subdermal cavity." When the former is the case, the 

 canal in some instances, as before noticed ('Annals,' /. c), goes 

 direct to the subjacent ampullaceous sac ; but as the latter 

 are much more numerous than the pores and for the most 

 part deeply situated throughout the structure of the sponge, it 

 may be assumed that the original pore-canal sends off branches 

 to supply them respectively. On the other hand, when the 

 pore opens into the subdermal cavity, it may do so singly or 

 in variable plurality. 



In some instances the pores are not generally distributed 

 over the surface, but chiefly limited to certain cribriform 

 areas, each of which forms the summit of a prominent 

 pustular eminence. These eminences, although separated 

 from each other, are plentifully scattered over the surface of 

 the sponge ; and while the pores are open and in active 

 operation the pore-area thus formed presents an expanded 

 convexity, but when they are closed it is conical, puckered, 

 and contracted. 



In some instances, again, the dermal layer, together with 

 the subjacent sponge-structure, is prolonged into mastoid (teat- 

 like) or tubular appendages, which thus not only increase the 

 extent of the pore-areas, but specialize it, so as to indicate 

 that these parts in particular are appropriated to the inhalant 

 function. 



Subdermal Cavities. 



In 1857 ('Annals,' vol. xx. p. 25), in my account of the 

 development of Spongilla from the seed-like body, the " sub- 

 dermal cavities," as they are more or less united together, have 

 been termed the "cavity of the investing membrane" (/. c. 

 pi. i. fig. l,bb b). In 1864 Dr. Bowerbank directed attention 



