28 Mr. II. J. Carter on the Anatomy 



to this structure under the terra of " intermarginal cavities " 

 (B. S.vol. i. p. 100). 



The subdermal cavities are situated immediately under the 

 pores, which thus open into them; and presenting a much 

 more open or cavernous structure generally than that which 

 lies inside them, they are easily recognized in a section of 

 the sponge perpendicular to the surface, where they at once 

 point out the side on which the pores are chiefly situated, in 

 contradistinction to the opposite or vent-bearing surface, whose 

 margin is comparatively without them. 



The subdermal cavity has an hourglass-shape, in some 

 sponges at least ; and the constricted portion is furnished with 

 a sphinctral diaphragm of sarcode which still further divides 

 them into two chambers, viz. an outer one, which is imme- 

 diately under the pores, and an inner one, which is extended 

 canal- like into the sponge. (For illustrations of this in Pachy- 

 matisma Johnstonia, Bk., see 'Annals,' 1869, vol. iv. p. 12 &c, 

 pi. ii. figs. 9-12.) 



Being an essential part of the pore or inhalant system, they 

 of course exist in all sponges, although perhaps most strongly 

 marked in the siliceous ones ; while the dermal sarcode which 

 covers them, having, as before stated, the property of opening 

 or closing its pores, can by this sphinctral power convert the 

 subdermal cavities into closed or open chambers as required, 

 to say nothing of the more powerful sphincter of the hour- 

 glass constriction of the cavity itself, which may act in unison 

 with the pores, or as a check upon them when they admit 

 material that ought to have been rejected. 



How these cavities terminate inwardly — that is, whether, 

 after branching out, their radicles are directly, or indirectly 

 through the medium of the ampullaceous sacs, connected with 

 those of the excretory canal-system (to be presently described), 

 or whether some terminate one way and some the other — re- 

 mains to be shown. (See a description and figure of the sub- 

 dermal cavity, 'Annals,' 1869, vol. iv. pi. vii. figs. 15, 6 & 9.) 

 As the sponge increases by additional layers to its surface, 

 new subdermal cavities must be continually formed, as the 

 old ones become obliterated by passing into the more compact 

 areolar structure of the interior. 



Excretory Canal- System and Vents. 



The excretory canal-system commences in radicles among 

 the ampullaceous sacs, which radicles pervade the body of the 

 sponge and, uniting with each other plexus-like, form branches 

 that finally terminate in a large trunk, which opens on its 



