The term is unfortunate, since the epithelium of a mucosa does not necessarily produce mucus. It's the barrel-shape that makes it a goblet cell. Colonic epithelium has huge numbers of goblet cells. Supposedly for sensation, you'll never pick them out by light microscopy. They make serotonin and much else.
Nobody knows why.
The connective tissue of the mucosa. It's generally loose, rich in vessels and lymphatics, and contains plenty of lymphocytes. The layer of connective tissue lying between a muscularis mucosae and the major muscular layer. The nerve plexus in the submucosa of the gut. The major muscle layer of a hollow organ, giving its strength.