Stratified epithelium has more than one layer of cells. Pseudostratified epithelium is a single layer of cells so shaped that they appear at first glance to form two layers. Note the single layer of simple cuboidal epithelium lining either side of a tubule. I have outlined one of the columnar epithelium cells. Secretion of a product.
Glands can be single epithelial cells, such as the goblet cells that line the intestine.
Multicellular glands include the endocrine glands . Many animals have their skin composed of epithelium. Vertebrates have keratin in their skin cells to reduce water loss. Many other animals secrete mucus or other materials from their skin, such as earthworms do. Collagen fibers provide elasticity and flexibility.