The document summarizes the external coverings of various microorganisms, plants, invertebrates, and vertebrates. It describes that microorganisms like viruses have protective protein capsids, while euglena and paramecium have outer coverings called pellicles. Bacteria and archaea have cell walls containing peptidoglycan or other chemicals. Plants have protective epidermal and cuticle layers. Invertebrates like arthropods, echinoderms, and mollusks have exoskeletons, spines or shells. Vertebrates from jawless fish to mammals have skins or feathers that protect against water loss and provide scales, hair or nails.

1. Survey of External Coatings of Microorganisms, Plants, Invertebrates and Vertebrates

2. Microorganisms Viruses Viruses consist of genetic material—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—surrounded by a protective coating of protein, called a capsid , with or without an outer lipid envelope. The viral protective shell, or capsid, can be either helica l (spiral-shaped) or icosahedral (having 20 triangular sides). Capsids are composed of repeating units of one or a few different proteins. These units are called protomers or capsomers . Euglena They have a flexible outer covering known as a pellicle and a long flagellum that propels the body. Paramecium It has a tough outer covering known as a pellicle. About 2,500 tiny, hairlike projections, called cilia, extend from the pellicle; the cilia move back and forth like oars to help the paramecium move about. Bacteria & Archaebacteria The cell walls of archaebacteria and bacteria contain forms of peptidoglycan, a protein-sugar molecule not present in the cell walls of fungi, plants, and certain other eukaryotes. The archaebacteria cell wall has a more diverse chemical composition than the cell wall of bacteria.

3. Plants Plants The dermal system consists of the epidermis, or outermost layer, of the plant body. It forms the skin of the plant, covering the leaves, flowers, roots, fruits, and seeds. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and thus reduces water loss from the plant surface through evaporation Stems increase in length through the activity of an apical meristem at the stem tip. This growing point also gives rise to new leaves, which surround and protect the stem tip, or apical bud, before they expand. Apical buds of deciduous trees, which lose their leaves during part of the year, are usually protected by modified leaves called bud scales. Stems increase in diameter through the activity of lateral meristems, which produce the bark and wood in woody plants. The bark, which also contains the phloem, serves as a protective outer covering, preventing damage and water loss.

4. Invertebrates Arthropods Arthropods make up a phylum of invertebrates that includes insects, crustaceans, and arachnids. The bodies of all arthropods are covered by an exoskeleton, or cuticle, that contains chitin. This tough outer covering, against which muscles work, is secreted by the epidermis fused with it. The exoskeleton remains fairly flexible at specific points, allowing the exoskeleton to bend and and appendages to move. The exoskeleton protects arthropods from water loss and helps to protect them from predators, parasites, and injury.The one main drawback to this type of skeleton is that it does not grow along with the rest of the animal and must periodically be shed in a process called molting. Echinoderms Members of the phylum Echinodermata, commonly called echinoderms, or spiny-skinned animals, are so named because of their spiny outer body coverings. The skeleton, made up of calcium carbonate, may form a large proportion of the body or it may be greatly reduced. Mollusks Mollusk, common name for members of a phylum of soft-bodied animals (Latin mollus ,”soft”), usually with a hard external shell. Familiar mollusks include the clam, oyster, snail, slug, octopus, and squid. Most mollusks have a glandular body covering, called the mantle. In some mollusks, such as clams and snails, the mantle secretes a hard shell.

5. Invertebrates Annelids Annelids have a thin, horny cuticle pierced by pores through which epidermal glands secrete mucus. In some marine annelids, glands are also present that secrete materials constituting a parchmentlike or calcareous tube within which the worm dwells. Earthworms and leeches secrete cocoons from a specialized epidermis in a region of the body known as the clitellum. Sponges Sponges have a simple epithelium, known as the pinacoderm, which both covers the external surfaces and lines the internal waterways. Some sponges deposit needlelike spicules of calcium carbonate in the jelly (mesoglea) beneath this outer epithelium. Cnidarians In the cnidarians the epidermis provides all the basic features of an integument. It may contain not only epithelial cells, some of which may be contractile, but also gland cells, pigment cells, stinging cells, and sensory cells with projecting hairs. The outer surface often bears flagella or microvilli. Flukes and Roundworms The parasitic flukes have a relatively thick integument, which bears many spines and sensory papillae, an apical membrane that is thrown into ridges and pits, and microvilli. The outer part of the integument contains secretory bodies, which are continuously released at the surface to renew the apical membrane.

6. Vertebrates Jawless fishes lampreys and hagfishes; have relatively thick skin; of several types of epidermal glandular cells; one secrete the protective cuticle. Multicellular slime glands secrete large amount of mucous to cover the body surface for protection. Cartilaginous fishes multilayered and contains mucus and sensory cells. The dermis contains bones in the form of placoid scales called denticles. Denticles contains blood vessels and nerves and is very similar to vertebrate teeth . Bony fishes contains scales. A thin layer of dermal tissue overlaid by the superficial epidermis normally covers scales. Because scales are not shed the grow at the margins and over the lower surface. The skin of bony fishes is permeable and can be used in gas exchange. Mucus are at help in fighting off bacterial and fungal infection at the surface. Amphibians consists of stratified epidermis and a dermis containing mucus and serous glands plus pigmentation cells. Amphibians are transitional between aquatic and terrestrial vertebrates. The earliest amphibians were covered by dermal bone scales. Reptiles reflects their greater commitment to a terrestrial existence. The outer layer of the epidermis (stratum corneum is thick, lacks glands and is modified into keratinized scales, scutes, plaques and beaks). The thick keratinized layer resists abrasion, inhibits dehydration and protects like a suit of armor.

7. Vertebrates Birds shows many typical reptilian features with no epidermal glands (only epidermal glands in birds is uropygial or preen glands). Over most of the bird’s body, the epidermis is usually thin and only two or three cell layers thick. The most prominent part of the epidermis are the feathers. Feathers are derived from the scales of reptilian ancestors and are, most complex of all derivatives of the vertebrate stratum corneum. Mammals notable feature include the a.)hair, b.)grater variety of epidermal glands than in other vertebrate class, c.)highly stratified cornified epidermis, d.)a dermis many times thicker than the epidermis. The prevention of dehydration is one of the evolutionary reasons mammals and other animals have been able to colonize terrestrial environment. The thickest portion of the mammalian skin is the dermis which contains blood vessels, lymphatic vessels, nerve endings, hair follicles, small muscles and glands. In humans and few other mammals like horses, the skin regulates the body temperature by opening and closing sweat pores and perspiring or sweating. The skin of humans and other mammals contains several types of glands: a.)Sudoriferous glands (sweat glands) secrete sweat by the process of perspiration which helps regulate body temperature and maintains homeostasis, b.)Sebaceous glands - glands connected to the hair follicles in the dermis. They lubricate and protect by secreting sebum. Sebum is a permeability barrier and emollient--skin softening and protective against microorganisms. Mammalian skin color is due to the pigments or to anatomical structures that absorb or reflect light. Melanin dictates the color of the skin.

8. Vertebrates Mammals Hair is composed of keratin filled cells that develop from the epidermis. The portion of the hair that protrudes from the skin is the hair shaft. And the portion embedded beneath the skin is the root. An erector fili muscle attaches to the connective tissue sheath of the hair follicle surrounding the bulb of the hair root. When this muscles contracts it pulls the follicle and its hair to erect position. In humans, this is called “goose bump”. Nails, like hair, are modification of the epidermis. Nails are flat horny plates on the dorsal surface of the distal segments of the digits.