Some protozoa have only a plasma membrane for external covering which is structurally and chemically identical to the plasma membrane of multicellular organisms. Plasma membrane has large surface area relative to body volume so that gas exchange and removal of soluble wastes occur by simple 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. It offers protection and is a semi rigid structure that transmits force of cilia to the entire body of Protozoa as it moves.
Most multicellular animals have single layer of columnar epithelial cells called epidermis. It rests on basements membrane. Beneath basement membrane is thin layer of connective tissue fibre and cells. Epidermal cells exposed at the surface of the animal may possess cilia. The epidermis of some invertebrates also contains glandular cells which secrete an overlying non cellular material that encases part or most of the animal.
Some invertebrates like rotifers have cuticles that are thin and support the body. Such cuticles consist of chitin and proteins in rigid plates by which flexible membrane links together. Cuticles retard growth. Thus some of invertebrates like arthropods periodically undergo molting. In Cnidarians like Hydra the epidermis is only few cell layers thick. Other cnidarians like corals have mucous glands that secrete calcium carbonate (CaCO3) shell. In outer covering of parasitic flukes and take worms in a complex syncytuim called tegument.
Nematodes and Annelids have epidermis that is one cell thick and secretes a multilayered cuticle.
In others like crustaceans, Arachnids and Insects cuticles are thick and rigid and support the body. Such cuticles consist of chitin and protein in rigid plates that a flexible membrane links together.
Disadvantages of Cuticles are that animals have difficulty to grow within them. So Arthropods undergo molting. Arthropods are made up of cuticular exoskeleton.
In Cnidarians such as Hydra epidermis is only few cell layers thick. Other cnidarians like corals have mucous glands that secrete calcium carbonate shell.
Tegument is outer covering of parasites and tapeworms which is complex syncytium. Its main functions are nutrients ingestion and protection against digestion by host enzymes.
Integument of echinoderms consists of thin usually ciliated epidermis and underlying connective tissue dermis containing calcium carbonate. Below cuticle is layer of ciliated epithelium which extends over spines, pedicellarue, tube feet and gills. Epidermis composed of ordinary flagellated or ciliated columnar cells, neuro sensory cells mucous gland cells or goblet cells with finely granular contents uniform gland cells filled with spherules and pigment granules which give colouration to the animal. Around the body of Molluscs is fleshy mantle which secretes a calcareous hell. The shell is usually external, though it may be internal, reduced or absent. Shell may be one piece or involve or two parts and known as bivalve.
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