SILICIOUS AND HORNY SPONGES WILSON. 483 



the fibers, near the end (pi. 52, fig. 5), or in cross sections of the 

 fibers. The fibers have a very definite sharp contour, as if the outer- 

 most laj'er were smooth and hard, making a kind of cuticle. The 

 spongin, which serves as cement, has itself a sharp contour, but 

 this is not as "cuticular" in appearance as the surface layer of 

 the fibers, although the difference is one of degree. At the free end 

 of the fiber (pi. 52, fig. 5) the outer layer is no more cuticular in 

 appearance than is the case with the spongin cementing the fibers 

 together. The substance of the fiber has the color and appearance 

 of spongin, appearing distinctly yellow when a sufficient thickness 

 of material is looked at. 



When the sponge is macerated for some time in cold caustic potash, 

 some of the connectives remain unbroken, the substance appearing 

 translucent and homogeneous: the medulla is made more distinct 

 and often looks granular; it may be locally widened and very 

 conspicuous (artifact). More often the connectives are split, more 

 or less, into shreds and tatters by the potash; the substance may at 

 first sight seem to be fibrous, but this appearance is due to the split- 

 ting and to the laminated structure of the substance. The sharp 

 contrast between outer layer and inner substance disappears in such 

 macerations, the pieces of a split fiber all looking alike, firm and 

 yellow, presenting the same appearance as the synapticula of 

 spongin. There thus can be no doubt that the connectives are made 

 up of spongin. 



After boiling in weak caustic potash (5 per cent) for a minute 

 or two, some of the connectives are changed in a significant way. 

 The medulla is made more distinct and is shown to widen out a 

 little at the end of the fiber; the surface layer is wrinkled, indicat- 

 ing that the inner substance is softer than it; at fractured ends the 

 inner substance may protrude in unbroken condition for some little 

 distance beyond the outer "cuticular" layer. In fibers broken 

 across in this way, the fractured surfaces of the inner substance 

 stain easily with haematoxylin, whereas with the same stain and 

 time the unbroken fiber stains scarce at all. All these appearances 

 indicate that while the fiber is made up of spongin the outermost 

 layer is especially hard and dense. 



After being just brought to a boil in strong (50 per cent) nitric 

 acid, the distinction between surface layer and inner substance of 

 the connectives disappears, and the whole fiber tends to split up as 

 in cold caustic potash, the outer layers sometimes peeling off from 

 an inner core. Boiling for a couple of minutes in this strong acid 

 indicates that the fibers are less resistant than the synapticula; the 

 former, in my preparations, had been very largely dissolved, while 

 plenty of synapticula remained. 



