CLADE or Class Hydrozoa: 3,000 species. Usually both polyp and medusa .

These have simplest body plan of all Cnidarians. Gastrovascular cavity is always simple. No cnidocytes in gastroderm.

 Most are dimorphic, with polyp and medusa generally of equal importance although there are orders in which either polyp or medusa is much reduced, or even absent entirely.

The "typical life style"


Medusae are missing in the life cycle of Hydra.

A polyp-like Actinula larva develops from the planula in orders Narcomedusae and Trachymedusae. This develops into the adult medusa, so there is no polyp stage as such in these orders.

In some, a medusa exists but it is retained.

Most are colonial with horizontal stolons connecting polyps. Rootlike stolons form a network of hydrorhizae which anchor colony to substrate. Polyps often differentiate into gastrozooids and gonozooids. There are sometimes also specialized defensive polyps (dactylozooids). All of these may occur within a single colonial organism, a so-called polymorphic colony. The dactylozooids and most gonozooids do not feed.



1. Hydra (Hydra littoralis)

link to review

The New Latin name, Hydra, is derived from the monster in Greek legend with many heads who grew two more whenever one was cut off. It was finally slain by Hercules who cauterized each neck after severing its head. The regenerative powers of hydra are quite remarkable, a single hydra may be cut into many pieces and provided that each piece contains a portion of the two body layers, ectoderm and endoderm, it will develop into a complete animal. A Hydra was first described by Leeuwenhoek in 1703.

Unusual because in Hydra there is no medusa.


Hydra can reproduce sexually. Look for ovaries near the base, testes higher on the column. Once fertilized, this egg develops a protective ornamented shell and frequently enters a stage of arrested development or dormancy. When favorable conditions return (often in the following spring), development resumes and the young hydra develops directly without a larval stage.





Common species has symbionts. Chlorohydra viridissima

So, although a common laboratory animal Hydra is an atypical hydrozoan, because it only has a polyp stage and lives in fresh water.

2. Craspedacusta sowerbyi

In other fresh water forms the medusa dominates. Craspedacusta sowerbyi, only common freshwater jellyfish.

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This species is more typical in that it has a polyp and a medusae.

As expected the medusae reproduces sexually. Egg and sperm are released and the fertilized egg or zygote develops into a planula larvae that eventually settles down and forms a poly.

There is a twist on this life cycle however because the polyps reproduce asexually, to form "frustules". They move slowly.


Frustule versus larvae size and structure

Also, during winter months, polyps contract and become dormant, as to survive the cold temperatures. These polyps, also known as podocysts, can be transported by aquatic plants, aquatic animals, or birds' feet to new areas of dispersal. As winter ends, and conditions become more favorable, the podocysts mature back into polyps.

3. Also most hydrozoans are colonial and show polyp polymorphisms. Most at least have gastrozooids and gonozooids. Obelia is an example. Also transparent protective tube, a perisarc, found in most species, composed of polysaccharides, protein and chitin. If it covers the polyps, it is known as a theca.

So Obelia is a thecae species, Hydra is athecae.

4. Interesting polymorphisms.

a. Velella has floating polyp colonies.

One of the individuals becomes a float. The float contains a series of sealed air chambers that provide buoyancy.  Total width of the floating polyp is usually less than 6 cm.  Beneath the float is a grouping of several types of zooids, colored brown by the presence of zooxanthellae.  A large central mouth (gastrozooid) is surrounded by shorter reproductive stalks (gonozooids) with mouth openings that bud tiny adult medusae that produce eggs and sperm.  Multitudes of tiny brownish-green medusae that never grow to more than 3 mm tall are cast off (last photo).  These then release the eggs and sperm that produce free-swimming larvae which eventually develop into more floating polyps

But there are alternate interpretations.

b. Physalia Portuguese Man of War appears to be a true colony.


CLADE or Class Scyphozoa: "True" Jellyfish. Medusa dominant, polyp reduced.

Jellyfish is a sea animal with a soft oval almost transparent body, includes other creatures including some snails sometimes. Certainly includes all the medusae and some floating polyp state of the Hydrozoa. Taxonomically the term is most used in conjunction with the large medusae of this class and Cubozoa

In polar waters some species can reach bell diameters of 8 ft. and have tentacles 130 ft. long.



Aurelia is a representative form. It may reach a diameter of 1 m. Aurelia has 4 trailing mouth lobes (oral arms) which each contain a ciliated groove and many nematocysts.

 The scyphozoan gastric cavity is divided into 4 gastric pouches that contain a tentacle-like projection of gastrodermis called a gastric filament which contain numerous gland cells that secrete digestive enzymes. Unlike the hydrozoans, the gastric pouches contain nematocysts.


Aurelia medusa diagram

Scyphozoans lack the marginal nerve ring of the hydrozoa. There are two (or more) nerve nets with ganglia, one which serves the whole surface of the animal, including the subumbrella, and one that serves only the subumbrella. The latter conducts impulses more rapidly and is concerned with swimming. They also have rhopalium, a complex of sensory structures (one statocyst, two simple eyes, and two chemosensory pits).



*****Below is a Obelia medusa (a hydrozoan medusa) for comparison. Note the simpler anatomy of a hydrozoan medusa*******



Back to discussing the class Scyphozoa

The sexes are often separate (dioecious). Sperm are released from mouth of male into the water and are filtered from the water by the female and fertilization is internal. Embryos develop into planula larvae in specialized brood pouches in the folds of the mouth lobes. Planulas are ultimately released and attach to overhanging rocks where they develop into polyps (scyphistoma). Scyphistomas bud-off additional polyps which ultimately undergo strobilation, a progressive constriction into a stack of saucer-like medusas (ephyras) which subsequently separate. Strobilation does not occur in the other Cnidarian classes and so the classes are distinguished by life cycle differences more than any other varying characteristic (6.9).

scyphistoma---------- strobila------------ ephyra


The Cubozoa are a class which are generally small and transparent.

They are sometimes described as the Cubomedusae or an order of scyphozoans (depends on book) . The bell is almost perfectly square in cross section. They have the most complex and sensitive rhopalial eyes of any Cnidarian (with a lens, retina, and thousands of sensory cells) and are strongly positively phototactic. Common names are "box jellies" and "sea wasps", named respectively for their square cross section and their powerful sting. They are the strongest swimmers among the Cnidaria and prey largely on fish and shrimp. Chironex, the most dangerous of any medusa known (tropical waters of north Australia) has a sting which causes intense pain and which has killed humans within 3 minutes. It get as big as a basketball with up to 60 tentacles hanging down as long as 15 feet.

Have the most complex of the rhopalia found among the "true" jellyfish.

They differ from Scyphozoa in that they have a velum like structure, the velarium, the bell has four flattened sides and a simple margin, and each polyp produces a single medusa by complete metamorphosis. The velarium restricts the size of the ventral opening through which water is expelled,which increases the force with which water leaves the bell.



CLADE or Class Anthozoa

Anemones and corals. Polyp only. (about 6,500 spp.) These are all marine. There are no fresh water anthozoans.


 The CLADE or Order Actiniaria are the "sea anemones". All are large solitary polyps with the typical polyp organization of column, oral disk, mouth, tentacles, basal disk. While most anemones are sessile, a few can burrow into soft substrates and a few are pelagic, using a floating device secreted by the basal disk.

 Since they are mostly sessile polyps, they lack the more complex nervous systems and structures seen in some of the medusae, but they do have at least one nerve net.

 The tentacles occupy the oral disc around the mouth. Mouth communicates to gastric cavity through a pharynx. Although external symmetry is radial, the internal symmetry is biradial or bilateral. The mesoglea is thick and a true cellular layer.



Another diagram emphasizing the mesenteries.


Gastrovascular cavity is divided by numerous septa (called mesenteries) which increase the surface area for absorption of the products of digestion. One mesentery is shown on each side of the animal, and others radiate around the coelenteron. The gametogenic tissue is embedded within the mesenteries between the musculature and the filaments. Nematocyst-studded acontia protrude from the interior edges of these mesenteries. The sphincter muscle around the margin of the oral disk is used to contract the oral disk. The anemone affixes itself to the substrate using its pedal disc. They have both longitudinal and circular "muscles" that can be used to bend the animal. As your book indicates in an emergency, all the longitudinal muscles can be contracted while the mouth is open. The animal becomes very flat and almost unrecognizable as a anemone. (6.21c) There can be fierce competition for space among individual with some fighting with specialized tentacles or bulges at the pedal base that contain highly poisonous nematocysts.


Reproduction: (a) asexually by spontaneous binary fission or fragmentation of whole animal (very unusual for a fairly complex metazoan). or (b) sexually. Depending upon the species, the sexes may be separate or hermaphroditic. The gonadal tissue is in the gastric septa. Eggs and sperm usually ejected through mouth and fertilization is generally external, although some species fertilize internally and brood their young either internally, or attached to the outer surface.

Of special interest to scientists is the mutualistic relationship that exists between anemones and certain fishes such as clown fish.



Order Scleractinia: Contains the corals, very important economically and ecologically.


Corals contain symbionts that produce photosynthetic products and aid in depositing calcium carbonate. Exact mechanism is unknown. Page 119. But rate of calcium carbonate deposition is greater in light than darkness. Symbionts gain access to CO2 and other nitrogenous products of digestion produced by anthozoans and protection.

The coral reef structure buffers shorelines against waves, storms and floods, helping to prevent loss of life, property damage and erosion. When reefs are damaged or destroyed, the absence of this natural barrier can increase the damage to coastal communities from normal wave action and violent storms. In fact, each meter of reef protects an estimated $47,000 of property value. Without coral reefs, parts of Florida would be underwater.

In fact, coral reefs support more species per unit area than any other marine environment, including about 4,000 species of fish, 800 species of hard coral and thousands of other species. Scientists estimate that there may be another 1 to 8 million undiscovered species of organisms living in and around reefs!

Coral reefs may become the medicine cabinets of the 21st century. Coral reef plants and animals are important sources of new medicines being developed to treat cancer, arthritis, human bacterial infections, heart disease, viruses, and other diseases. The pharmaceutical value of coral reefs in Jamaica's Montego Bay, for example, has been estimated at approximately $50 million. In text on page 126. there is a graph that shows the number of corals thought to be protected by the chemicals they produce.

Specific examples of potential benefits of reefs.

Reef sponges have been used in antiviral drugs to treat HIV and herpes

Chemicals from multiple reef species may be able to help fight cancer;

Sea fans are used to make products to relieve sunburn; and

The coral Pseudoterigorgia elisabethae, known for its anti-inflammatory properties, has been used in facial masques, cleansers and creams.

Coral anatomy is similar to that of the anemone

There is additional vocabulary associated with the calcium carbonate skeletons that are secreted.

The corallite is the part of the skeleton deposited by one polyp. The skeletal wall around each polyp is called the theca. Other structures include the calice (the upper opening of the corallite), the coenosarc (the coral tissue that stretches over the surface of the coral between the polyps), the coenosteum (the skeletal material around the corallites), and the corallum, which is the skeleton of the coral



The skeletons of stony corals are secreted by the lower portion of the polyp. This process produces a cup, called the calyx, in which the polyp sits. The walls surrounding the cup are the theca, and the floor is what we have identified as the basal plate. Thin, calcareous septa (sclerosepta), which provide structural integrity, protection, and an increased surface area for the polyp's soft tissues, extend upward from the basal plate and radiate outward from its center. Periodically, a polyp will lift off its base and secrete a new floor to its cup, forming a new basal plate above the old one. This creates a minute chamber in the skeleton. While the colony is alive, CaCO3 is deposited, adding partitions and elevating the coral. When polyps are physically stressed, they contract into the calyx so that virtually no part is exposed above the skeletal platform. The diversity of reef corals, i.e., the number of species, decreases in higher latitudes up to about 30° north and south, beyond which reef corals are usually not found. Bermuda, at 32° north latitude, is an exception to this rule because it lies directly in the path of the Gulf Stream's warming waters.


Coral reef distribution

Our existing coral reefs are fairly new environments.

Major stresses to coral reefs are: natural forces that they have coped with for millions of years; direct human pressures, including sediment and nutrient pollution from the land, over-exploitation and damaging fishing practices, engineering modification of shorelines; and the global threats of climate change causing coral bleaching (map below show hotspots), rising sea levels and potentially threatening the ability of corals to form skeletons in more acid waters. Hotspots for bleaching (yellow)n are indicated in yellow in map below.


Usually mass coral bleaching is noted during the time of the annual maximum in water temperature. And years that reach higher maximums are definitely associated with greater incidence of bleaching. The correlation is quite strong, and suggests that heat stress is at least part of the picture. However there are enough irregular details to conclude that there must be at least one more underlying causative factor as well.

"There are some correlations between the widespread coral bleaching in 1997-1998 and one of the strongest El Nino events of this century, but the patterns are unclear with many exceptions. The correlation exists for the east Pacific, but the bleaching in southeast Asia coincides with a strong La Nina (the complete reverse of El Nino), and the bleaching in the Indian Ocean and parts of the Caribbean do (not) correlate with either El Nino or La Nina." (Wilkinson, 1998--online NOAA report)

So, since it appears that corals are disappearing at an alarming rate in recent times, their susceptibility to bleaching is suspected to be a result of some combination of human activities, such as fishing and runoff from other activities. Reproduction: Corals start their life as a free-swimming young (after spawning, the planulae larvae is only the size of the head of a pin) that are carried by ocean currents. Planula larvae are ciliated and up to 1.6mm long. Most already contain zooxanthellae when released from the parental polyp and can survive in plankton for up to 100 days. The larvae will drift with the current until it finds a hard bottom to attach itself. . Once the larvae attaches to the bottom it quickly changes into a polyp (will never move again). It reproduces asexually by budding (in which an identical polyp sprouts out of the polyp's side).Through asexual reproduction, a coral can make a clone of itself. In this way, coral colonies are able to live for a few hundred years. Asexual reproduction is thus the main cause of colony growth.



Sexual reproduction is interesting since eggs and sperm in most cases released to the environment and so if the coral is not to self cross , must be synchronized. Many of the corals of Micronesia spawn (release their eggs and sperm) 7 - 10 days after the July full moon.

Research has shown coral eggs can distinguish among the different kinds of sperm present during the mass-spawning. They usually reject sperm which originate from their own parent colony but readily accept sperm from a different individual of the same species.

There are other groups of Anthozoa that are interesting but not as economically important. We look at some of these in lab, but confine our lecture discussion to anemones and corals.


What we see in Cnidaria and other animals:

True tissues with sheets of cells, epidermis as in most animals has basal lamina and a collagen fiber matrix. Regulation and protection of underlying tissue implied.


Have nerve nets and simple reflex responses and yet capable of some complex behavior.

Example: Hydra

Simple: Both the ectoderm and the entoderm have many muscle cells which contain small fibers that are able to contract. thus setting the body of the hydra in motion. In addition, there are nerve cells in the ectoderm; the cells located closest to the surface are receptors and the cells which are set deeper, among. the muscles, are effectors. If a hydra is pricked with a needle it squeezes itself into a tiny ball. This is a simple reflex caused by transmission of the stimulation from the receptors to the effectors.

More complex: After it has captured prey, the hydra uses its tentacles to draw the prey to its mouth and then swallows the prey. This behavior can also be explained by the aggregate action of simple reflexes connecting effectors and receptors locally. within small segments of the body. For example, the following model of a tentacle explains its ability to wrap itself around captured objects.

Model of a tentacle

Let us picture a certain number of links connected by hinges (for simplicity we shall consider a two-dimensional picture). Points A and B, A ' and B', B and C, and B' and C', etc. are interconnected by strands which can contract (muscles). All these points are sensitive and become stimulated when they touch an object (receptors). The stimulation of each point causes a contraction of the two strands connected to it (reflex).

No true differentiated muscle cells, but both epidermal and gastrodermal cells may have contractile extensions running in parallel along mesogleal surface (epithliomuscular cells or nutritive-muscular cells). That with a second nerve net devoted to more faster responses in some gives rise to pulsations of bell. Ineffective locomotion, but actually very effective in drawing food particles in.


Respiration by simple diffusion. Osmoregulation is still being investigated. Hydra can produce a hypo osmotic fluid in the gastrovascular cavity but the mechanism is unknown.

Digestion is extracellular, with secretion of enzymes into gastrovascular cavity by gastrodermis, then phago- and pinocytosis of resulting breakdown products. Some branching occurs in large forms. Undigested food expelled through mouth.


Asexual and sexual reproduction. Asexual mode used widely and impressively.





The body plan is very simple for many forms and symmetry is radial. These are two reasons why the group is not considered a suitable candidate for giving rise direct ancestor to biradial animals. Cnidaria are considered a successful side branch.

The anthozoans are considered the most primitive and the hydrozoans most advanced.

This is based on a cladistic analysis (Schuchert, 1993) and available molecular data. It would make the medusa the more derived stage. This seems consistent with the fossil record of the Cnidaria, but further studies will be needed to refine this picture of cnidarian evolution.

Cnidarians or protists?

Diagram above is of a species that parasitizes freshwater fish especially trout. It digests the fish cartilage leaving the fish deformed. Inflammation develops from the infection, puts pressure on the nerves and disrupts balance causing the fish to swim in circles, a disease known as whirling disease. Spores released from the decaying carcass may survive for up to 30 years in the mud. Spores are eaten by Tubifex worms which are eaten by fish.



Cnidarians form many symbiotic relationships with a variety of species.


Cloak anemone secretes a chitinous "shell" that surrounds the shell inhabited by hermit crabs. The anemone's shell grows as the hermit crab grows protection its growing abdomen.