Integumentary system of Protozoa:
Some protozoa have only a plasma membrane for external covering. This membrane is structurally and chemically identical to the plasma membrane of multicellular organisms. In protozoa plasma membrane has large surface area relative to body volume so that gas exchange and removal of soluble wastes occur by diffusion. This large surface area also facilitates the uptake of dissolved nutrients from surrounding fluids. Paramecium has thick protein coat called pellicle outside the plasma membrane. This pellicle is protective and is a semi rigid structure that transmits force of cilia or flagella to entire body of Protozoa as it moves.
Cuticle and tegument in invertebrates:
Some invertebrates like rotifers have cuticles that are thin and elastic. In crustaceans, archanids, insects, cuticles are thick and rigid and support the body. Such cuticles consist of chitin and proteins in rigid plates that a flexible membrane links together. Cuticles retard growth. Thus some of invertebrates like arthropods periodically undergo molting or ecdysis. In Cnidarians like Hydra the epidermis is only few cell layers thick. Other cnidarians (e.g. corals) have mucous glands that secrete calcium carbonate (CaCO3) shell. Outer covering of parasitic flukes and tapeworms in a complex syncytuim called tegument (i.e. to cover) host enzymes cannot digest it.
Amphibian skin:
The skin of Amphibians consists of stratified epidermis and dermis containing mucous glands, serous glands and pigmentation cells. Considering them physiologically they are transitional between aquatic and terrestrial vertebrates. The earliest amphibians were covered with dermal bone scales like their fish ancestor. The problems faced by amphibian due to the terrestrial environments are (i) desiccation (ii) the damage caused by ultra violet rays (iii) physical abrasion. Evolution resulted in increased amount of Keratin production in the inner layers of skin which napped to protect the cells from UV rays. Mucus produced by mucus glands performs three functions (i) prevents desiccation (ii) facilitates gas exchange and (iii) makes the body slimy. Some poison glands are found in the dermis which produce unpleasant tasting or toxic fluid which acts as a predator deterent. Sensory nerves penetrate the epidermis as free nerve endings.
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