Comparative anatomy of the animal body covering

INVERTEBRATE BODY COVERING

Protozoan body covering

·         Plasma membrane acts as the body covering.

·         Some have an additional gelatinous outer layer.

·         Some have a complex body covering called Pellicle. Ex:- Paramecium

Pellicle

·         Two membranes are present as outer alveolar membrane and outer alveolar membrane.

·         Between those two an alveolar cavity is present.

·         Some have cilia in the pellicle.

Poriferan body covering

·         Thin.

·         Two types of epidermis.

-          Pinacoderm: outer epidermis. Single layer of Pinacocytes. Pinacocytes are scale like falttened cells.

-          Choanoderm: inner epidermis. Single layer of choanocytes. Choanocytes are oval shaped cells with a flagellum on each.

·         Between those to lies the mesoglea or mesenchyma.

·         It’s a jelly like layer consist of spicules and amoebocytes, enclosing a central cavity called spongocoel.

·         Spicules supply strength and amoebocytes give rise to new cells.

·         Ostia are located on the wall.

Cnidarian body covering

·         Similar to that of Poriferan.

·         Outer epidermis: Cuticle is present.

                             Consist of small cubicle cells.

·         Inner gastrodermis: Mainly consist of columnar epithelial cells.

·         Mesoglea: Jelly like layer.

Platyhelminthes, nematode, annelid and arthropod body covering.

·         Single layer epidermis is present.

·         Cuticle is present on the epidermis.

·         Some arthropods have an inner layer of dermis.

VERTEBRATE BODY COVERING

·         Body covering is called as skin.

·         It has a multilayered structure.

·         Consist of two layers as epidermis and dermis.

Epidermis

·         Multilayered.

·         Avascular.

·         Forms structures such as feathers, claws, scales…etc.

·         From top to bottom of the epidermis;

-          Stratum corneum: Dead keratinized cells are on the surface.

-          Stratum germinativum

-          Basal lamina.

Dermis

·         Fibrous connective tissue with blood vessels, nerves and sensory receptors.

·         Fish scales are derived from the dermis.

Fish skin

·         Relatively thin.

·         Derivatives of the skin;

-          Scales: Bony scales derived from dermis.

             Bone cells come into the dermis and secrete scales. Afterwards they leave. Dentine and enamel layers are later deposited on those scales.

-          Melanophores: Star shaped cells that lie under the epidermis.

-          Glands

                                                                                     I.            Mucous glands: unicellular mucous producing glands. Mucous reduce friction when swimming. Produce mucous cocoon to protect from predators and heat in dry seasons. Also prevent invasion of bacteria.

                                                                                  II.            Poison glands: produce poisonous substances.

                                                                               III.            Light emitting glands: aids to attraction, species recognition and avoid predators.

Amphibian skin

·         Thin, moist skin.

·         Can breathe through the skin.

·         Derivatives of the skin

-          Glands

                                                                         I.            Mucous glands: keep the skin moist

                                                                      II.            Granular poison glands: parotid and parotoid glands.

-          Chromatophores: present in epidermis and dermis. Gives colorization.

·         No scales except for some species.

Reptilian skin

·         Thick, dry skin.

·         Stratum corneum is hardened with dead keratinized cells.

·         Can observe two layers in the epidermis.

-          Outer epidermal generation

-          Inner epidermal generation

·         Therefore they can shed scales or called as molting.

·         Scent producing glands are present in some.

·         No mucous glands.

Bird skin

·         Thin but horny scales are on legs and feet.

·         Claws and beak present.

·         Derivatives of the epidermis:

-          Feathers

                                                                 I.            Contour feathers: Cover the body.

                                                              II.            Flight feathers: Cover the wings. Large and stiffer.

                                                           III.            Down feathers:  lie beneath the contour feathers. Fluffy barbs.

                                                           IV.            Bristle feathers: Short and stiff but the barbs are absent.

·         Stratum corneum is not keratinized like in mammals.

·          Dermis lacks ossification.

·         Glands: mainly secrete lipids.

-          Uropygial gland: Single, branched alveolar gland located above the base of the tail. Produces fatty and waxy secretion. This is later spread over feathers to make waterproof.

-          Wax glands: lie in the external ear canal

Mammalian skin

·         Thick epidermis.

·         Transitional layers can be seen between stratum corneum and stratum germinativum.

·         Stratum corneum is thick and forms foot pads in many mammals.

·         Glands:

-          Sebaceous glands: alveolar branched glands that produce oil and wax.

-          Sweat glands: tubular coiled glands.

-          Mammary glands