Final answer:
Epithelial tissue is classified based on cell layers and cell shape, with cells being squamous, cuboidal, or columnar.
Step-by-step explanation:
The two criteria for classifying epithelial tissue are cell layers and cell shape. The shape of epithelial cells can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). The number of cell layers can be one layer, known as simple epithelium, or more than one layer, referred to as stratified epithelium. So, alongside cell layers, the classification of epithelial tissues depends significantly on the shape of the constituting cells.