PLATE LXXXI. 261 



sponge. The porous surface of the sponge also ex- 

 hibits numerous intermarginal cavities immediately 

 beneath the dermal membrane, but they do not exhibit 

 the large and cavernous appearance that we observe 

 in those of the exhalent surface ; nor do the connecting 

 spicula appear to be nearly so numerous. 



The oscula are exceedingly numerous towards the 

 marginal portions of the inside of the cup ; they are 

 comparatively large and are dispersed at unequal dis- 

 tances from each other ; towards the bottom of the cup 

 they are minute and are congregated in considerable 

 numbers above the large intermarginal spaces, the 

 largest of the oscula rarely exceeding a line in diameter. 

 The pores are comparatively speaking very large ; 

 there appears to be one only in each area of the dermal 

 network. They are very numerous, appearing upon 

 all parts of the outer or inhalent surface of the 

 sponge. 



The structure of the middle portions of the skeleton 

 is that of a very open and diffused halichondroid net- 

 work, the spicula being disposed in bundles of four or 

 five, or singly, in various directions, forming large 

 irregular angular areas. The mode of their disposition 

 is different near the surfaces of the sponge. They are 

 there collected in stout polyspiculous bundles, which 

 pass inwards at very nearly right angles to the surface, 

 and are connected with each other by widely spread 

 interstitial membranes, crowded with small tension, 

 and innumerable retentive spicula ; as these bundles 

 pass inward they merge in the central halichondroid 

 portion of the skeleton. 



The interstitial membranes are abundant and widely 

 spread in the interstices of the sponge ; they are fre- 

 quently crossed by single large skeleton spicula, but 

 their principal support is derived from a profusion of 

 minute subfusiformi-acerate tension spicula, which 

 cross each other in every possible direction. 



With a power of five or six hundred linear these 

 spicula exhibit a very incipient state of entire spination. 



