Zoology 10th Edition by Stephen Miller,John Harley

Exercise 5

Some invertebrates have just a plasma membrane, such as in the protozoa. Other invertebrates have a single layer of epithelial cells, some have cuticles, and some parasites have a tegument. Hair is composed of keratin-filled cells that develop from the epidermis. Nails are flat, horny plates on the dorsal surface of the distal segments of the digits. In addition, it is of interest to note with respect to integumentary systems that jawless fishes have a relatively thick skin. Cartilaginous fishes have a multilayered skin containing mucous glands. Bony fishes have dermal bone and scales. Amphibians have a dermis containing mucous glands and pigment. Nonavian reptiles have keratinized scales. Avian reptiles have no epidermal glands. Mammals have the most complex integumentary system, with many layers, functions, and associated structures. Sudoriferous glands produce sweat, whereas sebaceous glands produce oil.
What are some of the skin adaptations that occurred in amphibians as this group evolved a life divided between land and water?