2. Cnidaria: General Biology
The colorful cnidarians are the flowers of the sea.
Mostly marine (10,000 species); a few fresh water (20
species), but none live on land.
Cnidarians can be solitary but can be big (like jellyfish),
or colonial (like stony corals).
Most cnidarians are carnivores that catch and disable
prey with stinging tubules fired from uniquely
specialized cells called cnidocytes.
To humans, cnidarian stings create a burning sensation
like that of a nettle plant, after which Cnidaria is
named ( cnid- = nettle).
3. Form and Symmetry of
Solitary Individuals
• General
In basic form, the saccate cnidarian body
resembles a gastrula, which consists of a gut like
cavity enclosed by a solid body wall.
The cavity, called coelenteron, opens the exterior
via a mouth surrounded by one or more whorls of
tentacles. Sensory organs, when present, are
distributed at or near the bases of the tentacles.
4. • The cnidarian body exhibits radial symmetry around
one axis, the oral-aboral axis, that extends from mouth
to base. The oral end of the body, which bears the
mouth, is at one end of the axis, and opposite it is the
aboral end.
Radial symmetry may be useful when an abundant but
diffuse resource plankton, light- or danger has an equal
probability of arriving from any or all directions. This
applies to most cnidarians because they do not stalk
their food, but instead feed opportunistically on prey as
it inadvertently swims or drifts into contact with them.
8. Tissues and Body Compartments
The cnidarian body wall is composed of
three tissue layers: an outer epithelium,
the epidermis; an inner epithelium, the
gastrodermis, which lines the gut-like
coelenteron and joins the epidermis at the
mouth; and between these two epithelia,
a gelatinous extracellular matrix called
mesoglea.
10. A stolonate colony, like a strawberry runner,
consists of a prostate colon, single or branched in a
two-dimensional network, that attaches to the
surface of the substrantum and bears upright,
unbranched zooids. The stolon is a tubular body wall
outgrowth that includes the coelenteron, from which
zooids bud at intervals along its length.
Colonies of several taxa consist of zooids that arise
from the surface of a continous sheet of tissue called
a stolonal mat or coenosarc that unites the zooids
and attaches the colony to the substantrum.
11. The colonies described so far are more than
less simple, consisting of zooids borne either
directly on stolons like bulbs on a string of
Christmas lights or individually on a
coenosarc. More complex colonies arise not
only by the budding of zooids from stolons or
coenosarc, but also from other zooids. With a
few exceptions, zooidal budding produces an
upright, plantlike, fruticose colony that may
be grassy, bushy, shrubby, or feathery in
appearance.
12. Cnidaria: fruticose (shrubby) colony
A fruticose colony A fruticose colony can
resulting from also grow from an
axial polyp (stem-like),
fixed-length budding. which buds a succession
of lateral zooids.
13. Skeleton
• The startling variety of cnidarian skeletons,
especially evident in polyps, exceeds even that of
sponges. Exoskeletons may be the thin cuticles of
chitinous periderm, as found in many small-bodied
solitaty polyps and colonies, but not Hydra or most
anemones, which are naked. The polyps of stony
corals secrete a hard calcareous exoskeleton that in
some reef species can, over many generations,
reach a diameter of 1m or more and weigh several
tons.
15. Muscular and Movement
• The principal muscle of medusae is the halo-
shaped, circular coronal muscle on the subumbrelar
surface. The cross-strained coronal muscle is
antagonized by the elastic mesoglea.
• Cnidarian muscles are primarily epidermal and
gastrodermal epitheliomuscular cells, but in
anthozoans and scyphozoans, some of these cells
have abandoned the epithelia, entered the
mesoglea, and transformed into myocytes, or “true”
muscle cells.
16. Hydra: locomotion by
crawling or somersaulting
Hydras and some sea anemones can detach and shift locations by creeping
on their bases, by inchworm-like crawling, by somersaulting, or by floating.
18. • The cnidarian nervous system, like that of all
eumetazoans, consists of superficial sensory neurons
that monitor the environment, motorneurons that
activate effectors such as muscles or cnidocytes, and
interneurons that join the sensory receptors to the
motorneurons.
• In Cnidaria, interconnected neurons form a pair of
complex, two-dimensional nerve nets.
• Diffuse conduction is a characteristic of nerve nets.
19. A ganglion is a concentration of neurons
that serves as a brainlike integration center.
As such, it receives sensory input, integrates
that information with other inputs, and
generates motor output.
21. Cnidocytes And Cnidae
• A unique and defining feature of cnidarians is the
cnidocyte: a combined sensory-effector cell that
plays a central role in prey capture and defense. Each
cnidocyte houses a cnida, a fluid-filled membranous
capsule containing a long tubular invagination of the
capsule wall.
• Three general types of cnidae- nematocytes,
spirocytes, and ptychocytes.
22. Cnidaria: circulatory system
Pattern of ciliary circulation in the radial canals, ring canal, &
tentacles of a medusa in side view (A) and from above (B).
In (C) shows a pattern of circulation in a sea anemone,
including flows in the pharynx.
23. Coelenteron: Nutrition and Internal Transport
• The coelenteron is a blind, saclike cavity lined by
gastrodermis and opening to the exterior via the
mouth. In large polyps, the cavity may be more or
less partitioned by septa, which increase the surface
area of the gastrodermis. Among medusae, it is often
regionally specialized into a centeral stomach from
which radial canals extend to join a marginal ring
canal.
24. Regeneration and Clonal Production
• Cnidarians have an impressive ability to heal and
regenerate missing parts after injury. In Hydra, for
example, loss of the oral end is followed by regrowth
and differentiation of a new mouth and tentacles.
Living specimens of the common anemone Aiptasia
pallida can be dissected, pinned open for
observation, and then returned to an aquarium,
where they will heal after a few days. Medusae also
heal and regenerate missing parts after damage, as
do planula larvae.
26. Sexual Reproduction and Life Cycles
• Cnidarian adults are generally gonochoric, but
certain taxa such as stony corals, have many
hermaphroditic species. Germ cells originate in the
endoderm and generally grow and differentiate in
the gastrodermis, except in some hydrozoans, in
which they migrate into the epidermis.
• Further development leads to a planktonic larva
known as a planula. It’s epidermis is monociliated
and its gastrocdermis is often yolky.
28. Cnidaria: development, larva & life cycle
View
A-B cleavage starts at the animal pole
C-D bastula stages
E-F gastrulation establishes the 2 germ
layers, ectoderm and endoderm
G the planula larva
H the presumed primitive life cycle is:
polyp planula polyp.
30. Polyp Form
• The body consists of a tubular column
surmounted by a wide plateau, the oral disc. The
oval or slitlike mouth is in the center of the oral
disc and a whorl of tentacles occurs at its margin.
• In solitary individuals, the base of the column is
often expanded into a pedal disc that adheres to
the substratum. The mouth leads into a tubular
but laterally compressed pharynx that descends
below the oral disc and opens into the
coelenteron via an internal opening.
31. • Because of the pharyngeal compression, the mouth
and internal pharynx opening are elongated slits. The
two opposite corners of the mouth exted inward
along the length of the pharynx as ciliated grooves
called siphonoglyphs.
