Cnidarians
PHYLUM CNIDARIA
-RECALL THAT THE END OF THE ANIMAL THAT CONTAINS THE MOUTH IS THE ORAL END, AND
THE OPPOSITE END IS THE ABORAL END.
-RADIAL SYMMETRY IS ADVANTAGEOUS FOR SEDENTARY ANIMALS BECAUSE SENSORY
RECEPTORS ARE EVENLY DISTRIBUTED AROUND THE BODY.
-THESE ORGANISMS CAN RESPOND TO STIMULI THAT COME FROM ALL DIRECTIONS.
-9000 species
Characteristics
-diploblastic tissue level organization
-Between the epidermis and gastrodermis is a jellylike layer called mesoglea.
-One kind of cell is characteristic of the phylum
-Epidermal and/or gastrodermal cells called cnidocytes produce structures called nematocysts, which are
used for attachment, defense and feeding.
-A nematocyst is a fluid-filled capsule enclosing a coiled, hollow tube
-The cnidocyte has a modified cilium, called a cnidocil.
-Nearly 30 kinds of nematocysts have been described.
-Nematocysts used in food gathering and defense may discharge a long tube armed with spines that
penetrates the prey.
-The spines have hollow tips that discharge paralyzing toxins.
-Six or more kinds of nematocysts may be present in one individual.
-Most cnidarians possess two body forms in their life histories
-Polyp - asexual – sessile -Medusa - dioecious - free swimming
-Medusa swim by gentle pulsations of the body wall.
-The gastrodermis of all cnidarians lines a blind-ending cavity, called the gastrovascular cavity.
-This cavity serves for digestion, the exchange of respiratory gases and metabolic wastes, and for
discharge of gametes.
-All enter and leave the gastrovascular cavity through the mouth.
-The food of most cnidarians consists of very small crustaceans.
-Nematocysts entangle and paralyze prey contractile cells in the tentacles cause tentacles to shorten and
draw food toward the mouth.
-Certain gastrodermal cells, called nutritive-muscular cells phagocytize partially digested food and
incorporate it into food vacuoles, where digestion is completed
-They have a hydrostatic skeleton to aid in support and movement.
-A hydrostatic skeleton is water or body fluids confined in a cavity of the body and against which
contractile elements of the body wall act.
-In the Cnidaria, the water-filled gastrovascular cavity acts as a hydrostatic skeleton.
-Polyps use a variety of forms of locomotion, they may move by somersaulting from base to tentacles
and from tentacles to base again, or move in an inchworm fashion, using their base and tentacles as
points of attachment.
-Medusae move by swimming and floating.
-Cnidarians nerve cells may be the most primitive nervous elements in the animal kingdom.
-By studying these cells, zoologists may gain insight into the evolution of animal nervous systems.
Nerve cells are located below the epidermis, near the mesoglea, and interconnect to form a two
dimensional nerve net.
-This net conducts nerve impulses around the body in response to a localized stimulus.
-Because cnidarians have large surface area to volume ratios all cells are a short distance form the body
surface, and oxygen, carbon dioxide, and nitrogenous wastes can be exchanged by diffusion.
-Most cnidarians are dioecious.
-Sperm and eggs may be released into the gastrovascular cavity or to the outside of the body.
-In some instances, eggs are retained in the parent until after fertilization.
-Medusae are nearly always formed by budding from the body wall of a polyp, and polyps may form other
polyps by budding.
-Buds may detach from the polyp or they may remain attached to the parent to contribute to a colony of
individuals.
-Hydrozoans are small, relatively common cnidarians.
-One cnidarian class with freshwater representatives.
-Most hydrozoans have life cycles that display alteration of generation; however, in some the medusa
stage is lost, while on others the polyp stage is very small.
-Hydrozoans can be distinguished from the other cnidarians by three features
-Nematocysts are only in the epidermis; gametes are epidermal and released to the outside of the body
rather than into the gastrovascular cavity; and the mesoglea never contains amoeboid mesenchyme cells.
-CLASS SCYPHOZOA
-members of the class Scyphozoa are all marine and are called true jellyfish because the dominant stage
in their life history is the medusa.
-Unlike hydrozoan medusae, scyphozoan medusae lack a velum, the mesoglea contains amoeboid
mesenchyme cells, cnidocytes occur in the gastrodermis as well as the epidermis, and gametes are
gastrodermal in origin.
-CLASS ANTHOZOA
-Members of the class Anthozoa are colonial or solitary and lack medusae.
-They include anemones and stony and soft corals.
-Anthozoans are all marine and are found at all depths.
-Anthozoan polyps differ from hydrozoan polyps in three respects.
1. The mouth of an anthozoan leads to a pharynx, which is an invagination of the body wall that leads into
the gastrovascular cavity.
2. The gastrovascular cavity is divided into sections by mesenteries that bear cnidocytes and gonads on
their free edges.
3. The mesoglea contains amoeboid mesenchyme cells.