Lab 4: Poriferans and the Radiate Phyla
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Phylum Porifera
- Class Calcarea - Leucosolenia (an ascon sponge), Grantia (a sycon sponge).
- Class Hexactinellida - Euplectella (Venus' flower basket) – a leucon sponge
- Class Demospongiae - larger marine sponges, including commercial and bath sponges as well freshwater sponges such as Spongilla that produces gemmules; all leucon type.
Phylum Cnidaria
- Class Hydrozoa - Hydra, Gonionemus, Obelia, Physalia (Portuguese Man-O-War) Class Scyphozoa - Aurelia
- Class Anthozoa - Sea anemones (e.g., Metridium) and hard corals - Soft and horny corals (e.g., sea fans, sea whips, sea pens, etc.)
Phylum Ctenophora - comb jellies
The Phylum Porifera ("pore bearers") contains approximately 5,000 species of animals called sponges. Although multicellular, sponges have a very simple grade of organization, and the body consists of a loose aggregation of cells embedded in a gelatinous matrix. While sponges lack any real organs or true tissues, they do contain different types of cells including: epithelial-like cells called pinacocytes that cover the exterior and non-flagellated interior surfaces, flagellated cells called choanocytes that line inner chambers and canals, and amoeboid cells called archeocytes that move about in the mesohyl and can differentiate into other types of cells that form parts of the skeleton (composed of collagen and/or spongin fibers often embedded with calcareous or siliceous crystalline spicules). Sponges show radial symmetry or none. Although most species are marine, a few are found in freshwater. In terms of their ecology, few animals appear to eat sponges, probably because a mouth full of spongin and spicules is not too appetizing! However, a few reef fish and the hawksbill turtle feed exclusively on sponges. Sponges provide homes, however, for many organisms, particularly those anchored to coral reefs.
Sponges are perforated by many tiny pores called ostia that permit water to enter one or more oscula (singular osculum) that let the water exit. These openings are connected by a series of canals, some of which are lined by collar cells called choanocytes whose flagella maintain a flow of water through the sponge. Sponges have three basic body plans:
(1) Asconoid sponges are small, simple forms with a tube-shaped body.
(2) Syconoid sponges also have a tubular body whose wall is much thicker and highly folded into a series of incurrent canals that deliver water into radial canals lined with choanocytes.
(3) Leuconoid sponges have the most complex organization, which permits for an increase in body size. Clusters of flagellated chambers are filled from incurrent canals and discharge their water via excurrent canals into a central osculum.
Class Calcarea contains sponges with calcium carbonate spicules of three or four rays. All three body plans are seen in calcarean sponges. The group is entirely marine, and most species are dull-colored and less than 4 mm long. Representative calcarean sponges include Leucosolenia and Sycon (shown in the image above).
Sponges in the Class Hexactinellida are commonly called glass sponges because their six-rayed spicules are made silica. Often the spicules are cemented together into a roughly cylindrical skeleton 10-30 cm tall. All species are marine, living mostly in deep ocean waters. The best known example of a glass sponge is Euplectella (Venus' flower basket), the skeleton of which is shown on the above image).
In addition to many marine forms, the Class Demospongiae also contains about 150 species of freshwater sponges. Most of these species (like Spongilla shown above) lack any distinct symmetry and grow as encrusting organisms on hard substrates such as rocks, sticks, etc. During the onset of unfavorable periods (freezing, drying, etc.), freshwater sponges undergo asexual reproduction to produce internal buds called gemmules. Although the parental sponge may die and disintegrate, the gemmules are highly resistant to adverse conditions and will develop into new sponges when conditions improve.
The Phylum Cnidaria contains about 9,000 species of hydras, jellyfishes, sea anemones, sea fans, sea pens, corals, etc. Cnidarians are characterized by radial symmetry and the presence of stinging cells called cnidocytes that are used for the capture of prey and defense. Most species are marine, but hydras are freshwater forms. Cnidarians are have two true tissue layers (an outer epidermis and an inner gastrodermis) separated by a connective tissue layer called the mesoglea. The basic body plan includes a mouth that opens into a blind gastrovascular cavity lined with gastrodermis. Two body types are seen in cnidarians; the polyp is a sessile, cylindrical form in which the oral end (mouth and tentacles) is directed upward and the aboral end is attached to the substrate. The polyp form is seen in hydras and sea anemones. The medusa body type (seen in a typical jellyfish) consists of a free-swimming, bell-shaped form with the aboral end directed upward and the oral end directed downward. The nervous system of cnidarians consists of a diffuse network of neurons called a nerve net in which any direction of conduction is possible, a useful property for animals with radial symmetry.
The Class Hydrozoa includes a diverse array of cnidarians. The typical hydrozoan life cycle includes both polyp and medusa body types. Although most hydrozoans are marine and colonial, a number of solitary marine and freshwater forms are also known. Examples of marine species include Gonionemus (a marine hydrozoan jellyfish), Obelia (a sessile, marine colonial form) and Physalia (a marine, floating colonial form that is found in tropical waters off the coast of Florida) Examples of freshwater forms include Hydra (a solitary freshwater form that retains the polyp body) and Craspedacusta (a freshwater hydrozoan jellyfish).
Gonionemus
Gonionemus is a marine hydrozoan jellyfish that swims with its convex aboral surface directed upward and its concave oral surface facing downward. Note the thin ring of transparent tissue called a velum that encircles the margin of the bell; scyphozoan ("true") jellyfishes lack this structure.
Obelia
Obelia is a colonial, marine hydrozoan that contains both feeding polyps called hydranths and reproductive polyps called gonangia. During sexual reproduction, free-swimming male and female medusae bud off the reproductive polyps, releasing sperm and eggs that fuse to form a ciliated, planula larva that attaches itself to the substrate and begins to form a new colony through asexual budding.
Physalia
Physalia (the Portuguese man-o-war) consists of a floating colony of different kinds of individuals attached to a gas-filled float called a pneumatophore. The tentacles of Physalia (which can be several meters long) are well supplied with batteries of nematocysts that can inflict painful stings to those that encounter them! Sometimes these animals are washed ashore during storms (as shown on the image above), creating potential problems for bathers or even beach users!
Hydra
Hydra is a genus of simple freshwater animals possessing radial symmetry. They are predatory animals, using batteries of nematocysts on their tentacles to sting and capture prey. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions. Since they are rarely more than a few millimeters long, hydras are best studied with a microscope.
Craspedacusta
These tiny hydrozoan jellyfish have been found in calm waters of freshwater lakes, ponds, reservoirs and large rivers on all continents. Although not often seen, they can be particularly abundant toward the end of summer when they can be found floating near the surface.
In the true jellyfishes (most of which are found floating in the open ocean) the medusa is the dominant form, and the mesoglea (which is thick) contains cells as well as fibers. A structure called the manubrium (which contains the mouth) is usually drawn out to form four oral arms that are used in capturing and ingesting prey. The tentacles, manubrium and often the entire body surface are well supplied with nematocysts that can often give painful stings if touched.
The anthozoans ("flower animals") include about 6,000 species of sea anemones, corals, sea fans, etc. that all have the polyp body form. In terms of habitat, all are marine and most lived attached to rocks, shells, timbers, etc. Anthozoans are distinguished by the presence of partitions of the gastrovascular cavity called septa and a pharynx hanging down from the mouth into the gastrovascular cavity. Sea anemones have a cylindrical body with a crown of tentacles on an oral disc surrounding a slit-like mouth that leads into a pharynx. In the stony corals the polyps look like miniature sea anemones; they live in stony cups of calcium carbonate secreted by lower epidermal cells (acts as an exoskeleton) and normally hide during the day and come out at night to feed. Symbiotic algae called zooxanthellae live in the gastrodermal cells of most forms. These algae facilitate calcium carbonate deposition and get carbon dioxide and other nutrients from the coral polyps (and without them, the coral cannot survive).
Sea anemones
Corals
Sea fans
This small phylum contains about 90 species of marine organisms called comb jellies, all of which have biradial symmetry. Along the sides of their bodies are eight radially arranged rows of comb plates (longitudinal rows of fused cilia) that are used for locomotion. Two long tentacles contain structures called colloblasts that secrete a sticky substance for capturing prey. Bioluminescence is common within the group, and although they may be very small, some forms are remarkably beautiful!
This is a slide of a simple marine sponge called Leucosolenia. This tubular sponge has an ascon body plan in which water kept in motion by the action of flagellated choanocytes enters the central spongocoel through dermal pores and exits through a single osculum. The skeleton of Leucosolenia consists of calcium carbonate projections called spicules embedded in a tough protein called spongin.
- Incurrent canal
- Radial canal
- Spongocoel
This slide shows a cross section through the body of calcareous marine sponge called Grantia (also called Scypha in some books). This sponge demonstrates the sycon body plan in which the wall of the body has been folded into a series of internal and external canals. Water enters the sponge through a series of incurrent canals and passes through internal pores called prosopyles into the radial canals, which are lined with flagellated collar cells known as choanocytes. It is the action of these choanocytes that keeps water moving through the sponge. From the radial canals, water then enters the central spongocoel through pores called apopyles to exit through a single opening called an osculum.
1 - Incurrent canal
- Spongocoel
- Radial canal
- Incurrent canal
2 - Radial canal
- Spongocoel
- Radial canal
- Choanocytes
- Apopyle
This slide shows an internally-produced, asexual bud that is called a gemmule. All freshwater sponges (Class Demospongiae) produce such highly resistant buds, which allow them to survive unfavorable periods such as winter or droughts. The calcareous spicules projecting from the covering of the gemmule provide additional protection.
- Epidermis
- Gastrodermis
- Mesoglea
- Gastrovascular cavity
This slide shows a cross section through the body of Hydra, a small freshwater cnidarian. The simple organization of this hydrozoan polyp is reflected in the fact that the body is composed of only two layers, an outer epidermis and an inner gastrodermis which lines the central gastrovascular cavity. These two layers are separated by a thin gelatinous layer called the mesoglea.
- Hypostome
- Tentacles
- Epidermis
- Hydranths
- Gonangia
This is a slide of many stained specimens of Obelia, a colonial marine cnidarian that shows a distinct polymorphism in the organization of its members. Feeding polyps called hydranths bear tentacles armed with nematocysts, a mouth and a thin outer covering or hydrotheca. Gonangia or reproductive polyps consist of a stalk containing medusa buds surrounded by a thin membrane called the gonotheca. When mature, these medusa buds are released from the gonangium through a central opening called the gonopore. All individuals are attached to a main stem known as the hydrocaulus, which consists of a cylindrical tube of living tissue called the coenosarc covered by a thin outer membrane or perisarc.
1 - Hydranths
- Coenosarc
- Hydrotheca
- Tentacles
2 - Gonangia
- Medusa bud
- Perisarc
- Gonotheca
- Coenosarc
This slide shows a medusa from the colonial cnidarian Obelia. Although most of the life of this colonial hydrozoan is spent in the sessile polyp form, medusa buds produced within reproductive polyps called gonangia escape into the water column. These free-swimming medusae produce either eggs or sperm, which unite to form ciliated planula larvae that swim for a while and then settle to transform into a new polyp, thus completing the cycle.
This slide shows a cross section through the pharyngeal region of the sea anemone Metridium, an anthozoan cnidarian. Although the life cycles of anthozoans only include the polyp body form, these polyps differ in several respects from those of hydrozoans. For example, in anthozoans the mouth opens into a tubular pharynx rather than directly into the gastrovascular cavity, which unlike that of hydrozoans, is divided by many sheets of tissues called septa, or mesenteries. Some of these mesenteries (called primary septa) extend from the body wall all the way to the pharynx while others (secondary and tertiary septa) extend only part way into the gastrovascular cavity. Bundles of retractor muscles are found within the primary septa. When contracted, these muscles shorten the sea anemone. In Metridium a ciliated groove called the siphonoglyph extends down once side of the pharynx from the mouth. The cilia in this groove conduct water currents into the sea anemone.
Close-Up of the Pharynx
- Siphonoglyph
- Retractor muscle
- Primary septum
- Pharynx
- Gastrovascular cavity
This slide shows a magnified view of a cross section through the pharynx of the sea anemone Metridium. Note the ciliated groove called a siphonoglyph on the right hand side of the pharynx. The cilia in this groove conduct water currents into the sea anemone while those lining the rest of the pharynx conduct water currents out of the organism.
Close-Up of the Septa
- Primary septum
- Secondary septum
- Tertiary septum
- Retractor muscle
- Septal filament
- Gastrovascular cavity
This slide shows a magnified view of a cross section through the body wall of the sea anemone Metridium. Note the three types of septa (mesenteries) that separate the gastrovascular cavity. Primary septa go all the way from the pharynx to the body wall, secondary septa (which often bear septal filaments) go about half way across, and tertiary septa are but short projections of the gastrodermis.
Close-Up of the Body Wall
- Epidermis
- Mesoglea
- Gastrodermis
- Septal filament
- Primary septum
- Secondary septum
This slide shows a magnified view of a cross section through the body wall of the sea anemone Metridium. In these diploblastic organisms, the body wall consists of an outer epidermis and an inner gastrodermis separated by a jelly-like layer of connective tissue called the mesoglea.
This slide shows two discharged nematocysts from Hydra, a small freshwater cnidarian. These organelles contain a coiled thread-like filament that can be rapidly everted like a harpoon when triggered by contact with an object or a chemical stimulus. In many cnidarians, these nematocysts contain powerful venoms that can be used for defense or to capture prey.
This slide shows the ciliated planula larva that results from the union of egg and sperm from the scyphozoan jellyfish Aurelia. This nonfeeding larval stage quickly settles on a substrate and begins a transformation into the next stage called a scyphistoma.
This slide shows the polyp stage of development of the jellyfish Aurelia called a scyphistoma. This small, sessile individual has a mouth surrounded with tentacles that are used to procure food. After a period of time, the scyphistoma is transformed into a stage of development called a strobila, which consists of a series of discs that are stacked on top of one another like plates.
This slide shows the early strobila stage of the jellyfish Aurelia. This sessile larva contains numerous discs stacked on top of one another. Eventually, each of these discs will break free from the stack as free-swimming ephyrae.
This slide shows the late strobila stage of the jellyfish Aurelia. This sessile larva contains numerous discs stacked on top of one another. Eventually, each of these discs will break free from the stack as free-swimming ephyrae.
This slide shows an ephyra, which is the last stage of the complex life cycle of the jellyfish Aurelia. Such free-swimming individuals will gradually grow and change into adult jellyfish.
- 1. Manubrium
- Gonads
- Tentacles
- Radial canal
- Circular (ring) canal
- Statocyst
- Oral arm
- Ring canal
- Radial canals
- Gastric pouch
- Gonad
- Rhopalium
This slide shows a stained specimen of the comb jelly Pleurobrachia that has been embedded in a Lucite block. Although comb jellies resemble cnidarians in some respects, these animals are placed in a separate phylum called the Ctenophora. The group gets its name from the eight rows of fused cilia called comb plates that encircle the body. While some ctenophores have tentacles, they do not contain the stinging cells found in those of cnidarians.
- Ostium (opening into an incurrent canal)
- Radial canal
- Apopyle
- Spongocoel
- Osculum
- Bud
All sponges are capable of both sexual and asexual reproduction. One form of asexual reproduction is the formation of external buds (6) that can detach or remain to form colonies. Freshwater sponges produces internal buds called gemmules at the onset of unfavorable conditions (such as the drying out or freezing of the aquatic habitats in which they live). Although the parental sponge may die and disintegrate, these structures are highly resistant to adverse conditions and will develop into new sponges when conditions improve. Also, since sponges have tremendous powers of regeneration, broken parts often develop into fully functional new individuals. Sponges are also capable of sexual reproduction. Although most forms are monoecious, that is, they have both male and female organs in the same organism (which is well suited to a sessile existence), some forms are dioecious, or in other words, have separate sexes.
During sexual reproduction, eggs are fertilized by motile sperm that enter through the internal canals, after which the zygotes develop into flagellated larvae that break loose and are carried away by water currents.
- Epidermis
- Gastrodermis
- Mesoglea
- Basal disc (foot)
- Tentacles
- Discharged nematocysts
- Gastrovascular cavity
- Bud
- Ovary with egg
- Testis
Hydras attach to vegetation by a basal disc ("foot") that secretes a sticky substance. They then capture prey with tentacles lined with cnidocytes. Within these specialized cells are organelles called nematocysts that contain coiled thread-like filaments that can be rapidly everted like a harpoon when triggered by contact with a prey object, injecting venom that kills or immobilizes the prey. Once the prey is subdued, it is swallowed whole through the mouth (not shown on the model) and then digested in the gastrovascular cavity.
Although capable of sexual reproduction, hydras reproduce most of the year by producing external asexual buds that eventually pinch off at the base and become new individuals. Also shown on the model is an ovary containing an egg and a testis that produces motile sperm.
- Tentacle
- Pharynx
- Siphonoglyph
- Gastrovascular cavity
- Primary septum
- Secondary septum
- Tertiary septum
- Acontia
- Pedal disc