integumentary system of Invertebrates
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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