Sponge?

[MSirmon]

I found this and a larger piece between Mt Blanco and Dougherty Tx. It is super lightweight. It doesn't float but does take its time sinking. While cleaning I noticed weird shell like markings inside where a piece was broken out (prior to my finding it). I'm not sure what to make of it but it has the characteristics of a sponge. How far off base am I? 

[chg057]

I'm leaning away from a sponge. I think it just happens to be a rock with some vugs that have been naturally weathered away to give it the sponge appearance. In the third photo, it appears that a pretty good-sized cubic mineral was eroded from the specimen immediately to the left of the shell imprint. Could be a halite cast or a pyrite cube that fell out. It all depends on the formation and depositional system.