28.2 Phylum Cnidaria

• Sexual reproduction in sponges happens when gametes are generated.
• Spermozoa result from the differentiation of choanocytes and are ejected from the osculum, whereas oocytes arise from the differentiation of amoebocytes.
• The production of gametes may occur throughout the year in some sponges.
• Sponges may become hermaphroditic, producing oocytes first and spermatozoa later.
• Cross-fertilization and genetic diversity can be encouraged by the temporal separation of gametes produced by the same sponge.
• Spermatozoa can be carried along by water currents.
• The sponge is where the early development of flagellated parenchymula occurs.

• Sponges spend their lives attached to a fixed substratum.
• They don't show movement over large distances.
• Sponge cells are capable of rearranging their cells.
• Sponge cells spread on a physical support show a leading edge for directed movement under experimental conditions.
• It has been speculated that this creeping movement may help sponges adjust to their surroundings.
• The pattern of movement has been documented in laboratories, but it remains to be seen in natural sponge habitats.

• There is a video on openstax.org showing the array of sponges seen along the Cayman Wall.

• Almost all of the cnidarians are marine species.

• These cells are located around the mouth and on the tentacles and serve to repel or capture prey.

• The cnidocil projection on the outer wall of the cell is sensitive to touch.
• If the cnidocils are touched, the hollow threads can reach 40,000 times that of gravity.

• Animals from the Cnidaria have stinging cells.
• There is a coiled thread and barb in the nematocysts of conjugates.
• The thread, barb, and toxin are fired from the organelle when the hairlike cnidocil on the cell surface is touched.

• There is an animation showing two anemones engaged in a battle.

• The polyp or tuliplike "stalk" form and the medusa or "bell" form are the two different body plans found in Cnidarians.
• As adults, polyp forms have a single opening in the mouth and the stomach facing up.
• The mouth and tentacles are hanging down from a bell.

• The medusa and the polyp are the two different body plans.
• All cnidarians have two layers with a jelly-like structure.

• The medusa serves as the sexual stage and produces gametes in these species.
• Both body forms are diploid.

• The colonial hydroid Obelia has an example of cnidarian dimorphism.
• The first is adapted for capturing prey and feeding.
• In Obelia, the colon is connected through a coenosarc.
• The gonozooid is adapted for the asexual budding and the production of sexual medusae.
• The reproductive buds from the gonozooid break off and mature into medusae, which are either male or female.
• Meiosis occurs when sperm or egg cells are produced in the medusa.

• There is a separate origin of gonad and gametes.
• After fertilization, the gametes are released into the water and the zygote develops into a blastula.
• A new colony of polyps is formed when the planula becomes a single polyp after it swims freely for a while.

• The colonial form of Obelia geniculata has two types of polyps, which are adapted for capturing prey, and gonozooids, which asexually bud to produce medusae.

• An Obelia life cycle animation can be found at http://openstax.org/l/obelia.

• All cnidarians have two layers in their body that are derived from the embryo.
• The outer layer of the animal's skin is called the epidermis, while the inner layer of the animal's skin is called the gastrodermis.
• In the planula larva, a layer of ectoderm surrounds a solid mass of endoderm, but as the polyp develops, the GI tract opens within the endoderm.
• There is a non- living, jelly-like structure between the two layers.
• Cnidarians show the presence of differentiated cell types in each tissue layer, as well as the presence of intercellular connections.
• The development of organs or organ systems is not advanced in this phylum with the exception of statocysts and rhopalia.

• Nerve cells are scattered across the body and the nervous system is rudimentary.
• The medusa bell has a nerve ring around it that controls the action of the tentacles.
• There are two types of nerve cells: motor and sensory.
• The main signaling molecule in the primitive nervous systems is peptides.

• Despite the simplicity of the nervous system, it is remarkable that it coordinates the complicated movement of the tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.

• There is only one opening in the stomach that can serve as both a mouth and an anus.
• When food is taken into the stomach, the cells lining the stomach absorb the food and excrete it back into the stomach.
• Some digestion occurs.
• The body of the animal has a gastrovascular cavity that distributes nutrition throughout the body.
• It serves both the bicyle and the circulatory system.

• The lack of a circulatory system to move dissolved gases limits the thickness of the body wall.
• A number of canals help to distribute both Nitrogen and Oxygen in the cnidarians.
• Nitrogenous waste diffuses from the cells into the water outside the animal or into the stomach.

• The Anthozoa and the Medusozoa are the two monophyletic clades of the Cnidaria.
• The corals, sea fans, sea whips, and sea anemones are included in the Anthozoa.
• There are several classes of Cnidaria in the Medusozoa and the Hydrozoa.
• Both Cubozoa and Scyphozoa are included in the other clade.
• Both the Anthozoa and the Medusozoa have species with both polyp and medusa forms.

• Anthozoa includes sea anemones, sea pens, and corals, with an estimated number of 6,100 described species.
• Sea anemones can grow up to 10 cm in diameter and are usually brightly colored.
• Individual animals are attached to a food container.

• The sea anemone is shown in a diagram and photographed.

• The mouth of a sea anemone is covered with squids.
• There is a slit-like mouth opening and flattened pharynx.
• The structure of the pharynx makes the anemones symmetrical.
• Water can be directed into the pharynx by a ciliated grooves on the opposite sides.
• Up to two-thirds of the body can be reached by the pharynx before it opens into the stomach.

• The mesentery consists of a fold of tissue between sheets of skin.

• The smaller cavities coalesce at the pharyngeal opening.
• The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nitrogen and gas exchange, as well as additional mechanical support for the body of the anemone.

• Sea anemones feed on small fish and shrimp.
• The sea anemones have a mutualistic relationship with the crabs.
• The anemone gets food particles from prey caught by the crab, and the crab is protected from the predator by stinging cells of the anemone.
• Some species of anemone fish, or clownfish, can live with sea anemones because they build up an immunity to the toxins contained within the nematocysts and also have a protective mucus that prevents them from being stung.

• The structure of coral polyps is similar to that of anemones, although the individual polyps are usually smaller and part of a colony, some of which are massive and the size of small buildings.
• Coral polyps feed on planktonic organisms.
• Some anthozoans have a relationship with zooxanthellae.
• The relationship between zooxanthellae and modern corals gives coral reefs their colors and supplies both organisms with food and water.
• According to a new study, the complex mutualistic association began more than 200 million years ago.
• The health of coral reefs, which provide habitat for roughly one-fourth of all marine life, is dependent on the health of the symbiotic relationship between algae and coral.
• Corals are being affected by a trend in ocean warming that has caused them to expel their zooxanthellae and turn white.

• Anthozoans can reproduce asexually by budding or fragmenting, or sexually by producing gametes, if they choose to do so.
• The male or female gametes give rise to a free-swimming planula larva.
• The larva settles on a substratum and develops into a polyp.

• There are about 200 known species of marine jellies in Class Scyphozoa.
• The medusa is the most important stage in the life cycle of most species.
• The largest scyphozoan species, Cyanea capillata, can reach a size of two meters in diameter.
• Scyphozoans have a bell-like appearance.

• A diagram showing a Jelly's Morphology is shown in a photograph.

• There is a mouth opening on the underside of the animal.
• Scyphozoans live a solitary life.
• The diverticuli are sacs that are connected by the mouth.
• The bicyle may branch into radial canals in some species.
• Like the septa in anthozoans, the branched gastrovascular cells help to support the body of the animal.

• In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body with a nerve ring around the bell.
• There are pockets in the edge of the bell that hold clusters of sensory organs.
• The contractile force required to swim through water as well as to draw in food from the water is provided by the ring of muscles in the dome of the body.
• There are separate sexes for scyphozoans.
• The gametes are expelled through the mouth.
• The planula are formed by external fertilization and settle on a substratum.

• The medusa buds may be bud to form additional polyps or begin immediately.
• The planula can develop directly into the medusa in a few species.
• Sexual medusoid and asexual polypoid body forms are included in the life cycle of most scyphozoans.

• The medusa stage and the polyp stage are part of the lifecycle of most jellyfish.
• The medusa reproduces sexually, while the polyp reproduces asexually.
• Members of the Cubozoa are not as large as those of the Scyphozoa.
• There are similarities between the scyphozoans and the cubozoans.
• The arrangement of the tentacles is different between the two classes.
• In some cases, the bicyle may extend into the pedalia.
• The arrangement of nematocysts along the tentacles helps to subdue and capture prey.
• Cubozoans are the most venomous of the cnidarians.

• The animals have unusual eyes, including a cornea, lens, and retina.
• True organs are structures made from a number of interactive tissues.
• There are four clusters of eyes.
• There are four simple eye spots and two oriented eyes in each cluster.
• Cubozoans have extensive nerve nets but no brain, so how images formed by these complex eyes are processed is a mystery.
• The presence of eyes helps the cubozoans to be active and effective hunters of small marine animals like worms, arthropods, and fish.

• Cubozoans have different sexes and fertilization occurs inside the female.
• Planula can be released or developed inside the female.
• The planula becomes a polyp.
• Each polyps becomes a single medusa when they bud to form a colony.

• The Malo kingi has four muscular pedalia to which the tentacles attach, just like allcubozoan jellies.
• Irukandji syndrome is a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological symptoms.
• Two people in Australia are believed to have died from Irukandji sting.
• Most of the Hydrozoa are marine, although some freshwater species are known.
• The familiar Hydra has only the medusoid form in it's lifecycle, but most species exhibit both polypoid and medusoid forms.
• The medusoid form of Scyphozoa is called a scyphomedusa.

• These animals have a cylindrical form with a central vascis lined by the gastrodermis.
• There is a layer of mesoglea sandwiched between the epidermis and the gastrodermis.
• There is a mouth opening at the oral end of the animal.

• The Portuguese man o' war's float creates a pneumatophore with which it regulates buoyancy by filling and expelling carbon monoxide gas.
• At first glance, the complex superorganisms appear to be a single entity, but the reality is that even the tentacles are composed of zooids laden with nematocysts.
• The colony is made up of many hundreds of organisms, each specialized for a certain function, such as feeding, reproduction and defense.
• Although they are able to eat soft body marine animals, P. physalis lack the stomachs to digest their prey in the open water.

• The male and female colonies release their gametes into the water.
• A new colony is formed when the zygote develops into a single individual.

• A chain of zoids can reach up to 50 meters.
• The solitary hydromedusae is a hydrozoan species.
• The gonads of the hydrozoans are derived from the same tissue as those of the other cnidarians.

• The Portuguese man o' war and Velella bae are colonies that are part of the family Hydrozoa.

• By the end of this section, you will be able to describe the unique features of flatworms, rotifers, and Nemertea.
• The evolution of a concentration of nervous tissues and sensory organs in the head of the organisms can be seen in these phyla.

• The mouth opens into the alimentary canal when the blastopore, or the point of invagination, is present.

• Protostomes include acoelomate, pseudocoelomate, and eucoelomate.
• In pseudocoelomates, there is a pseudocoelom that replaces the solid mass of mesoderm, leaving little room for organ development.

• Eucoelmate Protostomes are schizocoels in which cells migrate into the blastocoel during gastrulation and form a solid mass of cells.
• The coelom develops within the cell mass.
• As we will see later in this chapter, a coelom can be formed by pinching off the invaginating primitive gut, or archenteron, and then forming a complete coelom.
• Eucoelomate can form its "true coelom" by either schizocoely or enterocoely.
• The process that produces the coelom is different, but the result is the same.

• The name "lopho-trocho-zoa" is used for organisms that possess either a lophophore feeding apparatus or a trochophore larvae.
• The lophophore is a feeding structure made of ciliated tentacles.
• A trochophore has two bands of cilia around it's top-like body.
• One or both of the defining structures may be missing from some of the Lophotrochozoa.

• When ribosomal RNA and other genes are compared, their placement with the Lophotrochozoa is upheld.

• Much more work is needed to resolve the cladistic relationships among the complex group.

• The flatworms are acoelomate organisms.
• The Polycladida, a group of flatworms, are thought to be related to the trochophore larvae.
• The polycladids and other flatworm groups have spiral cleavage.
• There is a phenomenon called "blastomere anarchy," in which a sort of temporary feeding forms is followed by a regrouping of cells within the embryo that gives rise to a second-stage embryo.
• The monophyly of the flatworms has been supported by analyses.

• The Catenulida is a small clade of over 100 species.
• The worms reproduce asexually.
• The formation resembles a chain but the offspring do not fully detach from their parents.
• The flatworms discussed here are part of the Rhabditophora.
• The parasites of trematodes and cestodes are important to humans.

• Flatworms have a lot of diversity.

• Flatworms have three tissue layers that give rise to their internal tissues.
• The secretory cells that produce mucus and other materials at the surface of the skin are mesenchymal cells.

• Because flatworms are acoelomates, there is a solid mass between the skin and the stomach.

• The flatworms are free-living.
• Parasitic form feed by eating their hosts' food.
• In such animals, the "mouth" is used to expel waste from the digestive system, and also serves as an anus.
• The gut can be a simple sac or highly branched.
• Digestion is a process of taking digested materials into the cells of the gut.
• The tapeworms are a group of parasites that don't have a digestive system.

• The system regulates dissolved salts and excretion of nitrogenous waste.
• The nervous system has a pair of nerve cords that run the length of the body.
• The nerve cell bodies at the end of the worm are associated with photosensory and chemosensory cells.
• There isn't a circulatory or a respiratory system with gas and nutrient exchange dependent on cell-to-cell junctions.
• This limits the thickness of the body in these organisms, which makes them "flat" worms.
• Female and male reproductive organs are the same in most flatworm species, and fertilization is internal.
• Sexual reproduction is common in some groups.

• The planarian is a flatworm that has a mouth and anus.
• The excretory system consists of flame cells and tubules on both sides of the body.

• The nervous system is made up of two nerve cords that run the length of the body.

• The relationships among members of the four classes of flatworms have recently been reexamined, with the turbellarians in particular now viewed as paraphyletic.
• The Polycladida, which contains the large marine flatworms, is the most familiar of the multiple orders of the class Rhabditophora.

• The classes were divided into four.

• Most free-living flatworms are marine polycladids, although there are a number of members from other orders in both environments.
• The flatworms' ciliated skin facilitates their movement.
• Some free-living flatworms are capable of remarkable feats of regeneration in which an individual may regrowth its head or tail after being severed, or even several heads.

• The life cycles of the monogeneans are simple and consist of a free-swimming larva attached to a fish.
• The host of the parasites is usually very specific.
• The worms may digest the host tissues, or they may simply eat the surface mucus and skin particles.
• Cross-fertilization is common because most monogeneans are hermaphroditic and the male gametes develop first.

• Humans are parasites of mollusks and other groups.
• The life cycles of trematodes involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs.
• The primary host is usually a fish and the secondary host is usually a mollusk.

• In the tropics, an estimated 200 million people are affected by various forms of schistosomiasis.
• The human enters the water and the metacercaria is released from the snail host.
• The parasites feed on red blood cells before they reproduce.

• Many of the eggs are released in feces and end up in a waterway, where they can cause problems for the snail host.
• The barb on the eggs can cause damage to the human host and allow other pathogens to cause infections.
• Most of the ill effects of schistosomiasis are caused by the parasites' eggs.
• Many eggs do not make the transit through the veins of the host for elimination, and are swept by blood flow back to the liver and other locations, where they can cause severe inflammation.
• The eggs may cause damage to the liver.
• Control is difficult in impoverished areas with unsanitary conditions and poor prognosis in people with heavy infections.

• The cestodes, or tapeworms, are internal parasites.
• Taenia spp., like other tapeworms, live in the gut of the primary host and remain fixed using a sucker or hooks on the anterior end of the tapeworm body, which is essentially a colony of similar subunits called proglottids.
• The excretory system with flame cells may be found in each proglottid.
• Because they are so long and flat, tapeworms don't need a digestive system to absorb nutrition from the food around them.

• At the end of the tapeworm, proglottids are "mature" and all structures except fertilized eggs have deteriorated.
• Cross-fertilization between different worms in the same host is the most common method of reproduction.
• The proglottids are released into the feces of the organisms after detaching from the body of the worm.
• The eggs are eaten by another animal.
• The intermediate host is usually the site of the juvenile worm's residence.
• The cycle is completed when the primary host consumes the muscle tissue.
• There are tapeworm parasites that can be transmitted by eating pork, beef, or fish.

• A pair of ciliated feeding structures are seen under the light microscope.
• The rotifers are placed in three different classes by one treatment.
• The "spiny headed worms" currently in Acanthocephala appear to be modified rotifers and will probably be placed into the group in the near future.
• As more evidence becomes available, the rotifers will continue to be revised.

• The pseudocoelomate body of a rotifer is large enough to fit in a Paramecium and consists of a head, a trunk, and a foot.
• The body surface is covered by a cuticle, which is rigid in some species and flexible in others.
• They have both Skeletal and Visceral muscles that are composed of single cells.
• The toes or extensions of the foot can make sticky material that can be used to adhere to surfaces.
• There are nerves in the body and a number of eyespots in the head.

• Two of the three classes of rotifer are shown.
• The animals in the center of this scanning electron micrograph are surrounded by jaws from rotifers.

• In freshwater and saltwater environments, rotifers are found.
• They are very important components of the aquatic food web because they are filter feeders.
• A rotifer's food is directed toward the mouth by the movement of the cilia.
• The masticated food goes into the stomach and then into the intestines.
• After being collected in a cloacal bladder, excretory waste is released out the anus.

• You can watch a video to see rotifers feeding.

• There are about 2,200 species of rotifers.

• Some rotifers exhibit sexual dimorphism and are dioecious.
• In many dioecious species, males are short-lived and smaller with a single testis.
• Haplodiploidy is a method of sex determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male.
• The bdelloids may be able to compensate for their genetics by borrowing genes from other species.
• Up to 10% of a bdelloid genome has genes imported from a related species.
• Some rotifer eggs can be protected from harsh environmental conditions.

• The illustration shows a rotifer.

• The ribbon worms or proboscis worms are referred to as the Nemertea.
• There are an estimated 900 known species of phylum Nemertea.
• nemerteans have been recorded in freshwater and damp habitats.
• nemerteans feed on worms, clams, and crustaceans.
• Some species, like Malacobdella grossa, have evolved relationships with mollusks.
• Commercial fishing of clams and crabs has been devastated by economically important species.
• Two species of fish are sold as bait.

• Nemerteans can be found in different sizes from 1 cm to several meters.
• They show symmetry and contractile properties.

• Because of their contractility, they can change their presentation.
• There are soft and unsegmented animals in the form of a flattened tube.

• The proboscis worm combs the sea floor for food.
• The species is part of the same family.
• The specimen was taken in the Ross Sea.

• The proboscis is located in the gut and serves as a harpoon.
• It is ornamented with barbs.
• The rhynchocoel is a fluid filled space that extends from the head to the gut in these animals.
• The proboscis can be extended by the contraction of the rhynchocoel and by the pull of the rear wall of the rhynchocoel.

• The Nemertean's body is shown.

• You can watch a video to see a nemertean attack a polychaete.

• The nemerteans have a well-developed digestion system.
• There is a mouth opening that leads into the foregut.
• The rectum is open via an anus and contains diverticular pouches.
• The gnaads are open through the genitals.

• The closed circulatory system of nezertans is derived from the coelomic cavity of the embryo, so they may be considered coelomic.
• A pair of blood vessels form a closed loop in their circulatory system.
• There are some species that have both a dorsal and cross-connecting vessel.

• The fluid contains cells, but they are often not visible.
• The blood cells of some species have yellow or green colors.
• The blood vessels are contractile, and movement of blood is aided by the contraction of muscles in the body wall.
• The circulation of fluids in the rhychocoel is more or less independent of the blood circulation.