Phylum Cnidaria

(a stinging thread)

Also known as Phylum Coelenterata (-hollow gut)

This phylum contains about 10,000 species worldwide.

They are mostly marine with a few freshwater species.

• Cnidarians are regarded as originating close to the basal stock of the metazoan line.

• They have a 700 million back history based on fossil records.

Defining characteristics are:

• 1. nematocysts (cnidae); complex intracellular organelles that secreted by the animal

• 2. Planula larva in the life cycle.

Larva is bilaterally symmetrical

Cnidarians include; 1. corals

2. sea anemones

3. Fresh water hydroids (Hydra),

4. Jellyfish

5. Portuguese man-of-warPortuguese war ship

• Physalia

Cnidarians have a basic radial symmetry.

They possess only 2 layers of living tissue;- ectoderm ; the epidermis*- endoderm; the gastrodermis*

*Muscle fibers in epithelia

They have a Gelatinous layer called mesoglea (non living) in between epidermis and gastrodermis.

Amoebocytes (living cells) in the mesoglea involve in food digestion, nutrient transport, storage and defense.

• The body is sac-like with an internal gastro vascular cavity called “coelenteron” and an opening called the “mouth”.

• (Open-ended tubular gut; no anus)

• The mouth surrounds a ring of tentacles.

• No head

• Body axis is oral- aboral

• Cnidarians are characterized by having stinging cells calledcnidoblast (nematoblasts).

• These cells contain cnidae (nematocysts), a fluid-filledproteinaceous capsule with an opening in one end. Insidecapsule is a long, spirally coiled hollow thread/tube.

Cnidocil is a cluster of modified cilia

in the cnidoblast through which

chemical and tactile stimulation perceive to

act on cnida

Upon stimulation, the hollow thread/tube shoots

out explosively from the capsule, and eject from

the lid turning inside out as it goes.

It contains toxins which are used in food capture

and defense.

The entire process requires only about 3 milli seconds

• Cnidarians are primarily carnivorous; but

• some soft coral feed on phytoplanktons

• Some obtain nutrients through photosynthesizing symbiotic algae

• They show tissue level of organization. Muscle fibres occur at the base of the epidermal and gastrodermal cells.

•

• They do not have circulatory, respiratory and excretory systems.

• A nerve net consisting of multipolar neurones are located below the epidermis near the mesoglea.

• Some of them have sense organs such as statocysts and oceli.

(Simplest animals to possess true nerve cells and sense organs)

• The body form of a cnidarian is constructed following two basic body plans, the polyp form and the medusa form.

(Polymorphism)

• Both forms have tentacles arranged around the mouth.

Polyp/Polypoid form Medusa/Medusoid form

Usually sessile Usually mobile

Vase/cylindrical shape Bell/umbrella shape

Mesoglea thin Mesoglea thick

Mouth at the upper surface Mouth at the lower surface

Tentacles occur around the

mouth

Tentacles occur along the edge

of the umbrella

Solitary or colonial Always solitary

Sometimes they build hard

or soft corals

Never build skeletons

Eg. Corals,

Hydras,

Sea anemones

Eg. Jelly fishes

• In some species only the polyp form is seen in the life cycle while in some others only the medusa form is seen.

• Still, in the life cycle of some other species alternation of generation is seen between the medusa and the polyp forms.

Only polyp - Hydra

Only medusa –

Physalia Aurelia (Jellyfish)

Typical cnidarian life cycle

Cnidarians are divided into four taxonomic classes.

• Class Hydrozoa – includes- hydras

portuguese-man-of war

Obelia

• Class Schyphozoa – includes-jellyfishes

• Class Anthozoa - includes- sea Anemones

corals

• Class Cubozoa

Class Schyphozoa –Cup animals

• Include few hundreds (about 200) species of cnidarians

• All are marine.

• Eg. Aurelia

Scyphozoans are characterized by the predominant medusoid stage (scyphomedusae)

It looks an inverted cup.

The polyp stage exists only as a short-lived larval stage.

(Scyphistoma larva)

Scyphistoma larva

• Scyphozoan medusae are big; 2 cm - 2 m in diameter (relative to the hydrozoan medusae)

• Most scyphozoans swim actively by contraction of muscle fibres and mechanical properties of mesoglea.

• Contraction of muscle fibre results the reduction of the volume of space under the bell and excess water expel

out. This effects animal to propel opposite direction.

Mesoglea is thick that supports the body structure like a skeleton.

It is firm gelatin.

Hence, scyphozoans are commonly known as Jellyfish.

Mouth locates at the end of a muscular cylinder-known asmanubrium.

• They lack a velum(a shelf of tissue projecting inward fromthe margin of the bell in hydromedusae - Hydrozoan)

• The medusae bell is circular in cross section and thetentacles are distributed around the circumference.

• Sensory organs distributed around the edge of the bell,called the rhopalia, contain the ocelli (simple eyes) andstatocysts used for determining spatial orientation.

Scyphomedusae always have 4 oral arms surrounding the mouth.

• Gastrovascular cavity of scyphozoans is developed as fluid filled gastrovascular canals.

• Gastrovascular canals connect to the mouth through the manubrium.

• Food particles captured by nematocysts on tentacles and oral arm-----ingested at the mouth--- through manubrium---- to stomach -----distributed among four gastric pouches.

• Wall of the gastrovascular pouch cansists short tentacles with nematocysts that secrete digestive enzymes.

• Partially digested food particles are phagocytized and digestion completes intracellularly.

• Circulation of fluid in the gut take place by movement of cilia lining the gastrovascular canals.

The life cycle of a typical scyphozoan (Aurelia)

The life cycle of Aurelia includes alternating generations of medusae and polyps

The sexes are separated.

The lifecycle of a typical scyphozoan (Aurelia)

• In medusae, gametes are released from the gastrodermal gonads into the gastric pouches.

• Ova remain in the coelenteron but spermatozoa exit via the mouth into the sea. They enter the mouth of a female and make their way to the gonad where they fertilize the eggs.

Notes

• Cleavage begins in the gastric pouches. Embryos are released from the mouth and brooded on the oral arms. A hollow, ciliated blastula develops and invaginates to become a gastrula.

• The gastrula, known as the planula larva, is capable of independent existence and leaves the mother to swim in the plankton.

• The planula locates a suitable hard substratum, attaches, and grows into a scyphistoma larva a solitary polyp form.

• The scyphistoma can feed and may survive more than one reproductive season.

Notes

• The sessile minute scyphistoma, reproduce asexually by budding or by formation of cysts (podocysts).

• Under suitable environmental conditions, scyphistomae undergo transverse segmentation and segment metamorphosis (strobilation).

• The segmented parts of the strobilating polyp (strobila) develop into medusae that eventually break loose and become free swimming young jellyfishes (ephyrae).

• The ephyra grows rapidly into an adult medusa, completing the life cycle.

Notes

Class Hydrozoa(Water animals)

• 2,700 species of cnidarians include in Class Hydrozoa

• Include marine and freshwater species

Eg. Hydras, Portuguese-man-of war,

• Most hydrozoans represent a polyp stage than a medusastage in the life cycle.

• Eg. Hydra - do not have a medusa

• But, some spend part of their lives as medusa and part as polyp stage.

• Eg. Obelia

gastrozoid

gonozoid

• Hydrozoan medusae are also known as ‘’jellyfishes’’ but, much smaller (few cms or even less)

• Gastrodermal tissues of hydrozoans do not have nematocysts. They are restricted to epidermis.

• Mesoglea do not contain cells.

• A shelf of tissues called velum extend inward from the edge of the swimming bell towards manubrium

• This help to eject water in a grater pressure from under the bell through a narrow opening when contraction of musculature

Class Anthozoa –(Flower animals)

corals, sea anemones

~6,600 species.Marine.

The anthozoans are a key feature of all marine environments, most notably in the form of coral reefs.

They are sessile and either solitary or colonial.

Lack a medusa stage entirely (Only polyp stage that look like flowers).

• The oral disc is encircled by numerous tentacles (in multiples of 6 or 8) placed at top of a cylindrical column .

• The body cavity (coelenteron) is divided by mesenteries(septa) also in multiples of 6 or 8.

• Anthozoans have a muscular pharynx leading from the mouth to the gut.

Coral polyp secrete a prototheca by its epidermal layer;first as a basal plate and then develop as radial folds and a rim as a thecal wall in between mesenteries.

• There are two subclasses within the Anthozoa: Hexacorallia

Octocorallia

Hexacorallia (Zoantharia)

• The hexacorals have;

- simple tentacles around the mouth in multiples of six

- and six pairs of internal mesenteries.

• Eg. - common sea anemones, Fungia;(They are solitary) and

- most are reef forming true stony corals (hard corals) and

thorny corals; (They are colonial)

• Fungia Sea anemony

Coral

Octacorallia (Alcyonaria)

• Octocorals have eight branching (pinate) tentacles

and eight mesenteries (septa). Colonial & soft corals

• Eg. Gorgonia (Sea fan), sea pens, sea pancies (Renilla).

Many show polymorphism.

Class Cubozoa - box jellyfish

• ~20 species.

• Marine.

A cubozoan medusa.

• The cubozoans are similar to the Scyphozoa but the bell issquare in cross section, with a velum-like structure calledthe velarium.

• The velarium restricts the size of the opening throughwhich water is expelled when the bell contracts, thusincreasing thrust and making them stronger swimmersthan the Scyphozoa.

• There are four clusters of tentacles, one at each corner ofthe bell.

• The rhopalia of the Cubozoa differ from those of theScyphozoa in possessing very complex eyes with lenses,corneas and retinas.

• The lens is capable of producing very sharp images, asgood as human eyes but the focal length is longer thanthe distance between the lens and the retina making boxjellyfish strangely far-sighted.

• Development is also different in the cubozoans.

• Each scyphistoma forms a single medusa via completemetamorphosis.

• Cubozoa includes the highly toxic box jellyfish, found intropical regions and often in swarms which can drift intobays, disrupting human activities.