The pellicle can be a single or multiple layer of cells which may or may not be modified.

The epidermis is derived from the endoderm

A cellular cuticle is secreted by some invertebrates over the epidermis for additional protection

The cuticle functions as an exoskeleton and has special cuticulin insoluble proteins, glycoproteins, and lipids.

Platyhelminths can have Syncytial teguments that help them resist external environment pressures as well as enhance nutrient absorption.

Mollusca have a mantle that secretes a hard protective shell

Leucophore cells create greens, blues, silvers, and golds.

Chromatophores change pigments not the structure of the skin.

Arthropods have a chitinous exoskeleton that tough through sclerotization and calcium salts which only need to be shed once in a lifetime.

The Epicuticle layer is made of chitin

The basement membrane creates the epicuticle and the procuticle.

Amphibians have thin skin filled with chromatophores and poison glands

For cutaneous respiration to occur, the skin has to be thin and moist. The skin is loosely attached to the body at some points.

The mucous gland secretes mucous to prevent external water loss.

The sebaceous gland secretes sweat which is the body's way of keeping cool

The stratum granulosum produces new cells.

The beta keratin can only be found in reptiles

The closer the cell is to the surface of the skin, the more keratinized it is.

The scales on the surface are composed of keratinized endodermis and is often supported by osteoderms.

The central bony core of claws and horns is covered by a vascularized layer of nutritive layer of keratin.

The feathers of birds are made of keratin that is only arranged in a specific way that produces color.

Melanosomes can be arranged in thin melanin layers.

The scales of fishes are embedded in the dermis while reptile scales are produced from the dermis

Bony scutes, seen in Sturgeons, are dermally derived external plates overlaid with horn.

The antlers of deers are extensions of their skull and are hollow.

The brighter the colors of a frog, the more poisonous it is likely to be

The brighter the colors of a frog, the more poisonous it is likely to be

Colors may be produced by biological pigments produced by surface patterns or by structural colors that selectively absorb colors.

Structural colors rely on nanostructures that scatter light so that waves of specific frequencies become more apparent.

Structural pigments are not inside the cell, it is the consequence of the structure.

Carotenoids have melanocyte that produces brown, black and earth tones.

A hydrostatic skeleton is a rigid muscular skeleton supported by internal fluid pressure.

A hydrostatic skeleton is composed of longitudinal muscles and circular muscles. When longitudinal muscles contract, the circular muscles relax which elongates the body.

Bristle like setae extend to anchor the animal and prevent it from slipping

Muscular hydrostat is a muscle with a lot of fluid and is highly flexible.

Larger animals do not have an exoskeleton because it costs too much energy.

The notochord is a hydrostatic endoskeleton whose function is to provide stiffness to the body

The notochord is always restricted to certain life stages and will be lost as the animal develops.

The pure fluid inside the notochord has fluid-filled cells

Cartilage is more flexible than bone but is heavier and denser

The matrix of cartilage does not have calcium

Cartilage can serve as the skeleton for some species or form surfaces in tendons.

Bone is a living tissue that contains inorganic keratin laid on a extracellular matrix that is composed of collagen fiber and protein-carbohydrate gel

The distal epiphysis is closer to the midline of the body

The cortical bone contains trabeculae which is made of several bone cells that constantly remodel bone.

Osteoclasts form bone while Osteoblasts reabsorb bone

Endochondral ossification starts with the formation of an ossification center

Unmineralized osteoid: Trabecular matrix

Mesenchymal cells: periosteum.

Compact bone develops below trabecular bone

Bone ossification that starts from a cartilaginous blueprint begins with mesenchymal cells differentiating into chondrocytes. After the capillaries penetrate the cartilage, the primary ossification center develops.

An osteon is the basic histological unit of compact bones and contains a Haversian canal.

The axial skeleton includes the limbs and the pectoral and pelvic girdles.

The relative size of bone to mass changes depending on the species.

All movements of muscles are reliant on contractile protein

amoeboid movement is reliant on the control of myosin.

The dynein arm is a protein that stabilizes the structure of cilia and flagella

Cilia and Flagella are composed of different structural patterns of microtubules, a central microtubule pair and nine other pairs with a dynein arm attached to them

Invertebrates, such as giant barnacles, have striated fast muscles and smooth muscles.

Insect fibrillar have flight muscles that can move the wings very fast.

Muscles can be arranged in different configurations and combinations, making almost any movement, such as lengthening, possible.

Bands of muscles have particular bands due to the arrangement of fascicles.

The endomysium covers the whole muscle while the perimysium covers 1 muscle fasciculus.

The sarcoplasmic reticulum is responsible for releasing calcium during muscle contraction

The M line is the attachment point for actin and titin

The sarcomere is a major contractile unit of a fibril bordered by two Z discs.

Actin is a thin globular-subunit that can hydrolyze ATP

Myosin is a thick motor protein with a head that binds to actin to start a contraction

The troponin complex and the tropomyosin are regulatory proteins.

The tropomyosin has to be activated by the presence of calcium in order for the troponin complex to change its shape.

The nebulin is a myosin-binding protein that makes sure that the muscle filament length is correct.

Titin is the attachment point for myosin and actin and gives the sarcomere its structure.

The sliding filament theory (SFT) states that by utilizing ATP, the myosin binds to actin and pulls it towards its midline when a contraction is triggered by an outflux of calcium.

Troponin and Tropomyosin change shape in response to calcium outflux toward the sarcoplasmic reticulum

A neuromuscular junction connects the nervous and muscular systems. Synaptic vesicles release acetylcholine into the synaptic cleft where it binds to receptors and triggers an action potential to spread through the sarcolemma,

Calcium binds to tropomyosin, causing it to change shape and influencing troponin to change its form.

Myosin forms cross-bridges with the exposed active site on actin and uses ATP to swing towards the sarcomere center.

Another ATP molecule is required for the myosin head to detach from the active site on actin.

Most of the energy used in muscle contraction comes from glucose.

Slow oxidative fiber has low myoglobin and mitochondria

Fast and slow twitch muscles operate on the same principle but their differences are caused by a different number in myoglobin.

Tendons store muscular energy when the tendon is being stretched. It then releases the energy as it recoils.

The epidermis of Amphibians is stratified squamous

Pigments are large molecules that reflect light rays.

When longitudinal and circular muscles contract alternatingly, a wave of contraction passes from posterior to anterior.