PHYLUM CNIDARIA The cnidarians (or coelenterates) contain ...

PHYLUM CNIDARIA 

The cnidarians (or coelenterates) contain approximately 9000 species and include such organisms as 

hydra, jellyfish, and sea anemones. Many are brightly colored. All are radially symmetrical. The phylum 

is known for beautiful colors and appearances. For example, sea anemones are considered by some to 

look like flowers in bloom. 

Cnidarians are also called coelenterates, an old name which refers to the relationship between 

cnidarians and ctenophores, the sea combs, the next group we shall study. The ctenophores and 

cnidarians make up the radiate phyla. The rest of the phyla are all bilateral in symmetry or radiate as a 

secondary characteristic. 

Cnidarians have a mouth which leads to a gastrovascular cavity. Tentacles, extensions of the body 

wall, surround the mouth and serve to capture prey and insert the food into the mouth. From the mouth, 

food enters a gastrovascular cavity. This allows for a more efficient means of feeding and digestion than 

in sponges. 

The body wall is composed of three layers: 

(1) epidermis - lines the outer surface, 

(2) gastrodermis - lines the gastrovascular cavity, and 

(3) mesoglea. 

The mesoglea may simply be a thin noncellular membrane or be developed to the extent of having a 

thick, fibrous, jellylike material with or without wandering amebocytes. 

Cnidarians have two forms or structural types: 

(1) the polyp form and 

(2) the medusa form. 

The polyp form is sessile and resembles a tube with 

an oral end bearing a mouth and tentacles pointing 

upward. The opposite end, the aboral, is attached to 

some substrate (surface).

The medusoid form resembles an umbrella with the 

concave side up and the convex side down. Tentacles 

are arranged around the edge. It is free swimming. The 

mesoglea in this form is well developed and gives rise to 

the common name jellyfish. Some animals only have the 

polyp stage, some only the medusal stage, and others 

have both stages in their life cycle. 

There are a few freshwater species, but cnidarians are 

mostly marine. There are three classes: 

(1) Hydrozoa - includes the freshwater Hydra, the marine Obelia, the Portuguese man-of-war 

(Physalia), and Gonionemus. 

(2) Scyphozoa - the common jellyfish as Aurelia. 

(3) Anthozoa - the sea anemones, sea pens, and corals. 

For discussion of structure, locomotion, excretion and osmoregulation, nutrition, and reproduction of the 

phylum, we will take a look at the freshwater genus Hydra. It has no medusal stage, however, it is 

readily available in high school biology labs and will serve as a basis of study for characteristics of the 

phylum. 

HYDRA 

There are six common species, five of which are in the United 

States. 

1. Structure - The body, along with tentacles, may reach 2 cm 

in length. They are generally attached at the aboral end. 

The oral end has a mound or cone with the mouth at the 

apex. There are generally six tentacles surrounding the 

mound. 

a. Epidermis - There are 5 principle cell types in the 

epidermis: 

(1) epithelio-muscle cells, 

(2) interstitial cells, 

(3) cnidocytes, 

(4) mucus secreting cells, and 

(5) sensory nerve cells.

(1) Epithelio-muscle cells - These are 

probably the most important type. They are 

columnar in shape but expanded at the 

base, the expansion forming most of the 

epidermal surface. There are 2 or 3 

extensions of the base, each containing a 

contractile fiber called the myoneme. 

myoneme 

(2) Interstitial cells - These are not 

exposed to the outside; instead, they 

are wedged between epithelio-muscle 

cells. These give rise to sperm and 

egg. 

(3) Cnidocytes - These are lodged 

between the epithelio-muscle cells or 

in with them. They are very abundant 

on the tentacles. The cells are highly 

specialized and contain stinging 

structures called nematocysts. The 

cell in control of nematocysts is the 

cnidocyte. The cnidocyte is an oval 

shaped cell with a nucleus towards the 

base. One end of the cell contains a stiff 

bristle called the cnidocil. The interior of 

the cell has a capsule containing a coiled 

tube. The capsule has a lid. 

Nematocysts are discharged from the 

cnidocyte and are used for: 

(a) anchorage, 

(b) defense, and 

(c) capture of prey. 

The nematocyst is expelled when a mechanical trigger of the cnidocil causes a chemical 

change in the cnidocyte. Hydrostatic pressure everts the capsule explosively and the 

nematocyst is discharged. Nematocysts may remain attached to the cnidocyte or be 

freed.

There are three functional types. 

(a) volvent - entangles prey 

(b) penetrant - penetrates prey 

and injects paralyzing toxin 

(c) glutinant - sticky type that 

anchors 

Experiments show about 25% of 

nematocysts are discharged when Hydra 

eats a single brine shrimp. They are 

replaced within 48 hours. 

(4) Mucus secreting cells - Abundant in 

basal disc. They are used to adhere to 

surfaces. 

(5) Sensory and nerve cells - Sensory cells are abundant in tentacles. Nerve cells are 

located next to mesoglea. 

b. Gastrodermis - There are two cell types found in the gastrodermis not found in the epidermis: 

(1) the nutritive - muscle cell related to the epithelio-muscle cell except it is usually 

flagellated and develops pseudopodia for feeding. 

(2) the enzymatic-gland cells - These are flagellated cells which secrete digestive enzymes 

into the gastrovascular cavity. Other cells in the gastrodermis are those found in the 

epidermis. Nematocysts are not found in the gastrodermis of hydras, but may be found in 

other genera. In some cases, symbiotic algae grow in gastrodermal cells, imparting a 

green color to the hydras. 

2. Locomotion - Hydras can extend or contract or bend from one side to another. By taking in water 

through the mouth, a relaxed hydra may reach a length of 20 mm. It can at the same token, 

contract to no more than 0.5 mm. Hydras are essentially sessile organisms, but they are capable 

of shifting their locations. In one case, they somersault with their tentacles and base to move. In 

another method, a gas bubble is secreted by the basal disc and the hydra floats to a new position 

on the bubble. One characteristic of hydras is a periodic contraction. Hydras are constantly 

extending and contracting at periods of contraction every 5 to 10 minutes in daylight and less 

during night hours.

3. Nutrition - Hydras are carnivorous and feed mostly on small crustaceans such as brine shrimp. 

Contact with the tentacles stimulates discharge of nematocysts that entangle and paralyze the 

prey. The tentacles then contract and pull the prey toward the mouth. Mucus secretions aid in 

swallowing. Enzymatic gland cells secrete a trypsin-like enzyme that begins the digestion of 

proteins until the prey ends up as a soup. This is extracellular digestion. Evidence shows that 

proteins are broken down only as far as polypeptides. Next, digestion proceeds intracellularly by 

the nutritive-muscle cells. The nutritive-muscle cells extend pseudopodia to engulf small 

fragments of tissue. Digestion is finished in food vacuoles. Undigested matter is ejected from the 

mouth by contraction of the body to expel the gastrovascular contents. 

4. Gas Exchange and Excretion - There are no special organs for gas exchange or excretion. 

Gases are exchanged through body surfaces. Nitrogenous wastes, most often in the form of 

ammonia, is excreted through body surfaces. 

5. The Nervous System - Hydras possess a primitive nervous system, the first encountered in the 

animal kingdom. Nerve cells are arranged in an irregular net called a plexus. It is located 

beneath the epidermis next to the mesoglea and is particularly abundant next to the mouth. Nerve 

cells have the typical dendrite, somite, axon arrangement, and synapses do occur. 

6. Reproduction 

a. Asexual - asexual reproduction is by budding. It is most common in the warm months of the 

year. Evagination of the body wall occurs, and the bud actually shares the gastrovascular 

cavity for a period of time with the parent. Eventually the bud detaches and becomes 

independent. Since budding is common, it is not unexpected Hydra have an ability for 

regeneration. In one experiment, a hydra is everted from inside out and after a period of time, 

the gastrodermal cells reorient themselves on the inner side of the mesoglea and the 

epidermal cells migrate to the outside. As in sponges, when you disassociate cells, they will 

reaggregate. 

b. Sexual - occurs primarily in the fall. Most hydras are dioecious. Germ cells originate from the 

interstitial cells which form testes or ovaries. Testes are located in the epidermis of the upper 

half of the stalk and ovaries are found in the lower half. This is particularly true of 

hermaphroditic forms. A single egg is produced in each ovary. As the egg enlarges, it 

ruptures the epidermis. The testis is a conical swelling with a nipple-like structure from which 

sperm escape.

Protandry is common. The fertilized egg undergoes cleavage and forms a chitinous shell. 

When the shell formation is complete, the encapsulated embryo drops off the parent and 

remains dormant through the winter. Spring environments set off a chemical change that 

soften the shell and a young hydra comes out and establishes itself. 

CLASS HYDROZOA 

Although there are a large number of Hydrozoans, many are small and seldom noticed. They are found 

attached to shells, wharfs, rocks, etc. Freshwater species do occur, in particular, the colonial 

Cordylophora and the medusoid Craspedacusta. 

There are three unifying characteristics of this class: 

(1) the mesoglea is never cellular, 

(2) the gastrodermis lacks nematocysts, and 

(3) the gonads are epidermal, or if gastrodermal, they are never shed into the gastrovascular cavity. 

Most species have both the medusoid and hydroid forms, and the hydroids are typically colonial. In 

describing individual polyps, the term hydranth refers to the oral end bearing mouth and tentacles. The 

hydrocaulus refers to the stalk. The colony is most often anchored to the substrate by root-like 

structures called hydrorhiza. 

Most hydroid colonies are only a few inches in length and variable in shape regardless of the growth 

pattern. Shapes range from tree-like to feather-like. 

Coloration ranges from pink to orange but it is not often noticeable due to small size. Even though the 

size is typically small, one genus Branchiocerianthus, a deep sea species, may reach a length of over 

two meters. 

Colonial Hydrozoans often have a further modification dealing with the inherent problems of size 

increase. The epidermis secretes a nonliving chitinous envelope around the organism. The protective 

shell is called the perisarc, and the living tissue it surrounds is the coenosarc. Sometimes the perisarc 

is limited to the hydrocaulus, but it may also surround the hydranth in which case it is called the 

hydrotheca. Obelia has a hydrotheca. In some species, the hydrotheca is open, and in other species, it 

may have a lid (operculum) that opens when the hydranth feeds and closes when it retracts. The 

presence of a shell around the polyp classifies it as thecate, while an animal without the shell is called 

athecate. Some species have forms that are thecate only along hydroids, as Hydractinia echinata, 

which lives on snail shells taken over by hermit crabs.

Hydroid colonies also exhibit polymorphism. 

All colonies are at least dimorphic. This 

means each colony consists of at least two 

structurally and functionally different types. 

The most conspicuous and numerous is the 

feeding polyp called the gastrozooid. It 

basically looks like a short Hydra. They 

capture and ingest prey with a great deal of 

extracellular digestion occurring in the 

gastrozooid. They feed primarily on 

zooplankton. The digested broth from the 

gastrozooid is then circulated to the common 

gastrovascular cavity where intracellular 

digestion takes place. Circulation is 

enhanced by contractions of the body. The 

gastrozooids will often double as defensive polyps but some forms have specialized defensive polyps. 

They are generally club shaped with numerous nematocysts and adhesive cells. 

A second type of polyp (always present) is the reproductive polyp called the gonophore. They may 

arise virtually any place in the colony. In Obelia, the gonophores are restricted to a specialized polyp 

called the gonozooids. It consists of a central stalk called the blastostyle. This is where medusal buds 

form asexually. The perisarc extension around the gonozooid is called the gonotheca. Each bud on the 

gonozooid is capable of developing into a complete medusa. This finally pinches off and swims away as 

a minute jellyfish. 

Some of the more unusual modifications of hydroids occur in the genera Porpita and Velella (sea float). 

They are colonial forms which float. They may reach the size of saucers. Each colony is composed of a 

highly modified gastrozooid where the aboral end is flattened and developed as a float. Gonozooids 

hang down among the central mouth of the gastrozooid and marginal tentacles.

The medusal stage is quite small in Hydrozoans. The 

epidermal cells of the exumbrella are modified to appear like 

squamous epithelia. The margin of the bell folds inward to 

form a shelf called the velum. Tentacles hang down from the 

margin of the bell, usually four in number. They are richly 

supplied with nematocysts. The mouth is found at the end of 

the tubelike extension called the manubrium. It may also 

possess nematocysts and is often frilled. The gastrovascular 

cavity consists of a central stomach with canals leading away 

like the spokes of a wheel. These are called radial canals. 

Another canal runs along the edge of the bell and is called the ring canal. A swelling called a 

tentacular bulb is formed where each radial canal joins each ring canal. All parts of the 

gastrovascular cavity are lined by gastrodermis. The mesoglea is always thicker in the medusae 

than in the polyps, but even here it is acellular. The medusae are carnivorous as the polyps, and 

they feed on many things including small fish. Food digestion is the same as the polyp. 

The medusal muscle system, although small, is more 

developed and specialized. The muscular system is 

restricted to the epidermis and is best developed 

around the bell margin and subumbrella. Here, 

muscle fibers form a radial and circular network. The 

velum is also highly contractile. When the fibers 

contract, the water is pushed from beneath the 

subumbrella and the organism is driven forward, 

exumbrella first. Swimming is usually vertical in 

direction with the animal moving upward several 

beats and then floating downward. Even though 

marine medusae are isotonic, buoyancy can be 

regulated to a degree by changing ionic content. In 

particular, it has been found that sulfate ion content is lower in the medusae than in sea water. 

Horizontal movement is most often at the whim of the sea currents. 

The nervous system is also more specialized in the medusa. In the bell, epidermal nerve cells are 

concentrated and organized as two rings, one above and one below the velum. The nerve fibers

connect with nerve fibers in the (1) tentacles, (2) musculature, and (3) sense organs. The lower ring 

regulates contractions and is considered to be the pacemaker. The margin of the bell has numerous 

sensory cells and it is also the site of two sense organs: 

(1) ocelli - light sensitive, and 

(2) statocysts - gravity sensitive. 

Ocelli are composed of patches of pigment and photoreceptor cells. They are typically found on the 

outside of the tentacular bulb. Some medusae are positively phototrophic, some negatively 

phototrophic, and some do not react with light. 

Statocysts are located in between the tentacles or associated at the base of the tentacular bulb. Walls 

of statocysts contain sensory cells that project into the lumen. Attached to the sensory cells are 

bristles that are in contact with calcareous concretions called statoliths. The statolith responds to 

gravity and stimulates the appropriate bristles. When the wrong bristles are stimulated, the muscles 

contract and propel the animal until they correct their alignment. 

All medusae reproduce sexually. Most species are dioecious. Eggs and sperm arise from interstitial 

cells that migrate to specific areas, usually in the epidermis beneath the radial canals. Fertilization is 

external in some and in the gonads in others. Fertilization 

results in the development of a planula larvae. 

CLASS SCYPHOZOA 

In this class, the medusa is the dominant form in the life cycle. 

The polyp form is reduced or restricted. The size of the 

medusa is often larger than the Hydrozoan medusa, ranging 

in size from 2 to 40 cm with one giant species having a 

medusa of over two meters. Coloration is often striking with 

orange and pink being common colors. Over 200 species 

have been described with the range from the Arctic to the 

tropics. Freshwater species do occur but the class is 

predominantly marine. Unfortunately for us, their favorite 

habitat is around beaches. With their abundant nematocysts, 

they can be quite dangerous and even cause death. Some

species, such as the tropical Chironex fleckeri, are more potent than the Portuguese man-of-war. 

The medusae are similar to the Hydrozoan medusae. The 

bell varies from a shallow indention to a deep indention. 

The margin of the bell is often scalloped to form lobes 

called lapets. A velum is absent except for one family, the 

tropical Cubomedusae. The manubrium is elongated into 

four frilly oral arms which aid in capture and ingestion of 

prey. Tentacles around the bell vary in number. In 

Aurelia, the tentacles are small and form a fringe around 

the bell. The oral arms, manubrium, tentacles, and even 

the umbrella may contain nematocysts. 

The mesoglea is thick and gelatinous as in the 

Hydrozoans but it is cellular with amebocytes. It constitutes a layer derived from mesoderm. The 

muscular system and locomotion are basically the same as Hydrozoans except that some 

fast-swimming forms are capable of controlling horizontal movement. 

The canal system of the gastrovascular cavity is 

more highly modified with the major differences 

being an extensive system of radial canals. 

Ring canals may be absent. Scyphozoans feed 

on all types of small animals but few feed on 

fish. Larval fish of several species actually use 

Scyphozoans for protection. Some species, as 

Aurelia, are suspension feeders. As Aurelia 

sinks, plankton becomes trapped on the mucus 

of the subumbrella. Cilia sweep the food to the 

bell margin where the oral arms carry food to 

the mouth. One species of Florida and the 

West Indies, Cassiopeia, rests upside down on 

the bottom of mangrove embayments. It has many secondary small mouths that open into the 

stomach. It also possesses zooxanthellae, and if adequate light is present, it can survive through

photosynthesis alone. Gastrodermal nematocysts are present and probably are used to finish off prey 

not dead. 

The nerve net is further modified from the Hydrozoans in that there are concentrations of neurons in the 

margins of the bell called Rosalie. They number from four to sixteen. Each has two sensory pits, a 

statocysts, and ocellus. One sensory pit is located on the exumbrella side and the other one on the 

subumbrella. 

Most Scyphozoans are dioecious. The gonads are located in the gastrodermis as opposed to 

epidermal gonads in Hydrozoans. They are located in gastric pouches within the gastrodermis. Egg 

and sperm break out and are discharged through the mouth. The planula larva settles and forms a 

polypod larva called the scyphistoma. Young medusae (ephyrae) are budded from the scyphistoma. 

A scyphistoma may live several years. 

CLASS ANTHOZOA 

This class has no medusal forms. It includes the 

animals recognized as sea anemones, corals, sea 

fans, and sea pansies. It is by far the largest class 

of cnidarians with some 6000 species. The polyp of 

Anthozoans is considerably different from the 

Hydrozoan polyp. The mouth leads to a tubular 

pharynx that extends over half way into the 

gastrovascular cavity. The pharynx originates as 

ectoderm which invaginates into the gastrovascular 

cavity. The gastrovascular cavity is divided into 

compartment or septa by mesenteries. The 

mesentery edges contain nematocysts. The gonads 

are gastrodermal and the mesoglea is cellular. One 

major difference is that the nematocysts of the Anthozoans do not have an operculum (lid).

Sea Anemones 

These are solitary polyps ranging in size from 1.5 to 

5.0 cm in length with diameter ranging from the size of 

a dime to a half dollar. One exceptionally large 

anemone, Tealia of the North Pacific, may have a 

diameter of a meter. They are often brightly colored 

with white, green, blue, orange, and red as common 

colors. They inhabit any type of coastal water but are 

abundant in tropical waters. They grow attached to 

rocks, shells, and submerged timbers and a few even 

burrow into mud and sand. Some live attached to 

jellyfish and several commensal forms are found on 

the shells of hermit crabs. Calliactis actually transfers 

its anemone to a new shell. The European hermit crab has an anemone that actually envelopes the 

crab’s shell and extends it so that the hermit crab never has to seek a new shell. Some crabs attach 

anemones to their claws and use them for defense. 

At the oral end of the column are 8 to several hundred hollow tentacles. The mouth is a slit-like 

opening which has at one or both ends a ciliated groove called the siphonoglyph. The cilia of the 

groove provide water circulation into the gastrovascular cavity. This flow of water probably: (1) 

functions as a hydrostatic skeleton, and/or (2) provides additional opportunity for the exchange of 

gases. The anemone has a collar formed from the body column that covers the animal when it retracts. 

Sea anemones have an epidermis that may be ciliated and even be covered with a cuticle. They feed 

on various invertebrates and even small fish. Those with short tentacles typically are suspension 

feeders. 

In the Red Sea and Indo-Pacific region, a fish of the genus Amphirion lives commensally among the 

tentacles of large sea anemones. The fish secretes a mucus that raises the threshold of nematocyst 

discharge. The anemone provides protection and some food left undigested to the fish while the fish 

protects against some predators, removes necrotic tissue and also by swimming, provides ventilating 

movements. Some anemones have zooxanthellae. 

Movement is accomplished by a slow, gliding movement of the pedal disc or by walking on its tentacles. 

Some will escape danger by detaching and trying a swimming motion. Some move by gas bubbles on

the pedal discs and some species are actually pelagic instead of sessile. 

Asexual reproduction is most often by either longitudinal fission or by pedal laceration. In pedal 

laceration, the anemone simply leaves part of the pedal behind as it moves, and the pedal grows into a 

new anemone. 

Sexual reproduction occurs in either hermaphroditic or dioecious species. Gonads are gastrodermal. 

Fertilization may occur either in the gastrovascular cavity or externally. A planula larvae is produced. 

Stony or Scleractinian Corals 

Although these are closely related to the sea anemones, these Scyphozoans produce a calcium 

carbonate skeleton. Some are solitary, but the majority are colonial with small polyps averaging from 1 

to 3 mm in diameter. However, the entire colony can become quite large. The polyps are similar to the 

sea anemones, but coral polyps don’t have siphonoglyphs. Expansion and feeding typically occur at 

night. Corals are both raptorial and suspension feeders. The skeleton is secreted by the epidermis of 

the lower half of the column as well as by the basal disc. The process produces a skeletal cup within 

which the polyp is firmly fixed. The cup is called the calyx, the walls of the cup the theca, and the floor 

of the cup the tabula. There is a septum that projects upward into the base of the polyp, folding the 

basal layers. As long as the colony lives, calcium carbonate is deposited beneath the living tissues. 

The purpose of the skeleton is two-fold: (1) provide a place to live, and (2) for protection. When the 

polyp contracts, very little projects above the skeleton for fish or other predators to eat. The polyps of 

coral colonies are interconnected, but attachment is lateral rather than aboral as in hydroids. The 

patterns of the various skeletons species are due mostly to the arrangement of the polyps in the colony 

and the growth patterns of the species. Some form flat or rounded skeletal masses, while others form 

upright and branching growth forms. 

The colony expands its size by the budding of new polyps. The buds may arise from the base of the 

old polyps or from the oral discs of old polyps. Sexual reproduction is similar to that of the sea 

anemones.

Octocorallian Corals 

Sea anemones and corals belong to the subclass Zoantharia. The remaining species belong to the 

subclass Octocorallia and include the animals known as sea pens, sea pansies, sea fans, whip corals, 

and pipe corals. There are a number of distinctive features that set the octocorallian corals apart from 

the others: 

(1) octocorallians have eight tentacles and they are always pinnate, 

(2) there are always eight complete mesenteries, and 

(3) only one siphonoglyph is present. 

They are colonial cnidarians and the polyps are small and similar to stony corals. The colony is 

connected by a mass of tissue called coenenchyme. Coenenchyme is actually mesogleal material 

which is perforated by gastrodermal tubes that are continuous with the gastrovascular cavities of the 

polyps. Amebocytes of the coenenchyme secrete the skeletal material, and thus, the skeleton of the 

octocorallia is internal. It may be composed of calcareous spicules or of a horny material.

PHYLUM CNIDARIA The cnidarians (or coelenterates) contain ...

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