Zoology Lab exercise No. 4 - Porifera and Cnidaria

Phylum Porifera

Phylum Cnidaria

I. Phylum Porifera - Sponges (text chapter 17)

Class Calcispongiae -Grantia

Class Demospongiae - Spongia (common bath sponge)

Class Hyalospongiae - Euplectella (Venus' Flower Basket)

Sponges are the simplest of multicellular animals. Their cells are loosely organized and do not form true tissues or organs; hence, they remain at the cellular level of organization. If forced through a sieve, the cells of a some sponges will reassemble to reform the sponge body.

See examples of sponges on display, and refer to structure of sponge in text book.

There are three classes of sponges which have different skeletal structures.

1. Calcispongiae have skeletons of calcium carbonate spicules. The spicules, which look like tiny hairs or spines, are loosely held together by the living tissue. This fragile structure limits the size of these sponges. In many sponges, the spicules will pierce the tissues of animals attempting to eat the sponge, causing irritation.

Using a dissecting microscope, examine a preserved specimen of the sponge labeled "Grantia." Look at a whole specimen and at one which has been cut longitudinally. On the side of either sponge are numerous, small openings, ostia, which serve as entrances for water and food particles moving into the sponge. At the top of the sponge is an opening called the osculum which opens into an interior cavity, the spongocoel. Canals which lead into the spongocoel are lined with flagellated cells called choanocytes, or collar cells. Water is drawn into the interior of the sponge by flagellar movement within the canals and exits through the top.

Food is trapped by the collars of the choanocytes and ingested by phagocytosis (the same method used by Amoeba). Digestive products are then passed on to the protective epidermal cells which cover the exterior surface of the sponge.

2. Demospongiae have soft, flexible skeletons. On demonstration is a dried commercial "bath" sponge. This was once a living animal, but has been treated to remove the living cells. What is left is the "skeleton" made of spongin (protein) fibers.

3. Hyalospongiae have intricate skeletons made of glass. Also on demonstration is a skeleton of a glass sponge, which is named for its silicon dioxide skeleton. It consists of silicon spicules which have been fused into this rigid structure.

Can you think of one advantage and one disadvantage (to the organism) of each type of skeleton?

1. Calcium spicules:

2. Spongin:

3. Silicon structure:

II. Phylum Cnidaria (text Chapter 18)

Class Hydrozoa - Hydrozoans (Hydra)

Class Scyphozoa - True Jelly fish (Aurelia)

Class Anthozoa - Corals and sea anemones (Metridium)

This phylum is named for the presence of stinging cells called cnidocytes. The cnidarians have bodies consisting of two thin, often transparent, layers (endoderm and ectoderm) with a gelatinous layer (mesoglea) in between. There is no true mesoderm and hence they are diploblastic (have two germ layers). Members of this phylum have a digestive system consisting of a sac-like gut, referred to as the gastrovascular cavity. It has only one opening: the mouth.

This is a very large phylum with a tremendous diversity of species. Most members of this phylum are marine, but there are a few freshwater representatives. Many cnidarians cycle between two stages which have distinctly different body forms (polyp and medusa). Others assume either the polyp or the medusa form exclusively. We will examine Hydra (a hydrozoan) as an example of this diverse phylum. Members of other classes are on demonstration.

1. Class: Hydrozoa

Members of this class include freshwater and marine species which exhibit both polyp (sessile) and medusa (free- floating) stages. In general, the polyp stage is the dominant stage (larger) and the medusa is usually small in this class.

Obtain a prepared slide of "Hydra with bud". The Hydra is a free-living freshwater cnidarian which exhibits only the polyp stage in its life cycle. In this respect it is atypical of an hydrozoan, but it is relatively abundant and we shall consider it as a typical polyp form. In this whole mount you can easily see through the body of the hydra and easily identify the body layers. At the oral end of the animal, you will see the mouth surrounded by several tentacles. If you look carefully along the tentacles, you can see clusters of cnidocytes which appear as regularly spaced bumps.

Returning to low power, find the large gastrovascular cavity on the interior of the organism.

The Hydra may reproduce asexually or sexually by producing gametes. The specimen you are examining has produced an offspring by budding it from the wall of the parent Hydra. Is this reproductive method sexual or asexual? _____

If living Hydra are available, continue with this section. If not, go to "Other Hydrozoans".

Obtain a live hydra and place it in a watch glass with a little water. I will contract into a ball when handled, but will relax again when left undisturbed for a few minutes. Some small crustaceans are available to feed the Hydra. Add a few of these, using a pipette, and observe the behavior of the Hydra. Does it appear to sense the presence of its prey. If you watch closely, you may be able to observe the hydra stinging and ingesting one or more prey organisms. How many do you think it could eat in one meal?

Other Hydrozoans:

Obtain a prepared slide of Obelia. This colonial hydrozoan will exhibit the typical polyp and medusa stage in one colony.

You will see a stalk-like organism supporting two types of individuals. You will recognize the feeding polyps (individuals) or hydranths by their hydra-like tentacles covered with cnidocytes. Some of the polyps have extended tentacles while others may have withdrawn into the chitinous covering, which sheaths the entire colony. You will see some large oval-shaped, closed sacs called gonangia which contain clusters of small, round medusae buds (stained red here) which are asexually budding from the tentacle-less, reproductive individual. The medusae are the sexually reproducing stage of this organism. See Fig.18-9 (p. 401) of your textbook for an illustration of the complete life cycle.

The Portuguese Man-of-War resembles a jelly fish (medusa) but is actually a colonial, floating polyp (refer to text for details).

2. Class: Scyphozoa (True Jelly fish)

The medusae of the scyphozoans differs from the hydrozoan medusae in that

Examine an adult (medusa) Aurelia which is on demonstration. The life cycle of this species is complex and involves several tiny polyp stages (see text for details) which are seldom observed in the field.

This class, which is exclusively marine, includes the largest of jellyfish. The medusae of the scyphozoans (scyphomedusae) differs from the hydrozoan medusae (hydromedusae) in that the hydromedusae are generally smaller and round in shape due to the presence of a shelf of tissue called a velum (see text fig.18-11). The scyphomedusae lack a velum. Did you see the velum on the hydromedusae of Gonionemus? If not, go back and look more closely!

Examine an adult (medusa) Aurelia which is on demonstration. Notice that these medusae have four horseshoe-shaped gonads which are located near the center of the medusa. Look at the margin of a medusa. You will see many small marginal tentacles. Look for much larger oral tentacles surrounding the mouth.

The life cycle of Aurelia is considered to be typical for this class so we will look at the various larval stages. The medusae have separate sexes. Male medusae shed their gametes into the seawater and they fertilize the female's eggs. Small ciliated planula larvae are produced which swim in the water and eventually settle out on the bottom.

Obtain a slide of Aurelia planula and observe its size and shape. Can you see any cilia? ______

In non-preserved planulae, cilia cover the entire body. Draw the planula larva. As the planula crawls along the bottom it grows and eventually metamorphoses into a second larval stage: the scyphistoma larva. Obtain a slide of Aurelia scyphistoma and observe its "hydra-like" shape. How many tentacles does it have? ___________ Like Hydra, it will attach to the substrate by a pedal disc. You will never, however, find Hydra living in marine waters. Where would you find Hydra? _________________ This scyphistoma stage of Aurelia is the polyp stage. They are usually attached to bottom rocks, oyster shells, pilings or other submerged objects.

Draw a scyphistoma larva. After they have fed for a few weeks, the scyphistomae undergo an asexual transverse division to produce a strobilated effect and hence the new larvae are now called strobila larvae. Obtain a slide of Aurelia strobila and draw it. How many transverse divisional planes can you count? _____________ Each of the "section" of the strobilus will asexually bud from the top end of the polyp and be freed into the surrounding seawater as an ephyra larva. Each strobilus, then, may bud dozens of ephyrae. Draw an ephyra stage of Aurelia. How many emerging tentacles does it have? ________ What is the symmetry pattern of this stage? ___________ The ephyra stage is an immature medusa. Can you see any other adult structures on the ephyra?

List them _______________________________________________________________

See figure 18-16 (page 405) in your text for a life cycle drawing.

Examine demonstration materials of the Scyphozoa and take notes. Keep in mind that these delicate creatures do not always preserve well and most have lost their natural colors.

Of all the freshwater ponds, lakes, rivers and streams you have seen, have you ever seen a medusa in any of them? _________ Most people believe that jellyfish are restricted to saltwater? Actually, they are not! One of our former General Zoology students collected Craspedacusta sowerbyii in Lake Sherando near Charlottesville several years ago. This freshwater medusa (the only freshwater species of jellyfish in the U.S.) appears sporadically across the entire U.S.

The medusae of Craspedacusta have a velum. Based on this observation, to which class of Cnidarians would this species belong?

3. Class: Cubozoa (Sea Wasps)

The class Cubozoa includes the most toxic of jellyfish such as Chiropsalmus of tropical waters. The toxin of this jellyfish paralyzes the nerves of the heart and respiratory system. Swimmers who accidently bump into the tentacles of this jellyfish may drown before reaching shore. The tentacles typically hang down from 4 paddle-like structures on the bell margin. Since these jellyfish are flattened on the sides (cube-shaped) they are easy to place in the Cubozoa. We do not have any representatives of this class for you to view, however the typical shape can be seen in figure 18-17 of your text.

4. Class: Anthozoa (Coral and Sea Anemones)

This large class includes the sea anemones and corals. All are marine and exhibit only the polyp stage in the life cycle, the medusa stage having been completely lost.

On demonstration are specimens of the sea anemone Metridium which have been dissected to show internal structures. Note the sac-like gastrovascular cavity. Refering to the illustration in your text, locate the following structures: tentacles, oral disc, pharynx, gastrovasuclar cavity, and gonads. In addition you will find septal walls or mesenteries dividing the gastrovascular cavity. Were such mesenteries present in representatives of the other two classes of cnidarians? ___________

Make a drawing of oral-aboral section of a sea anemone. Sketch the structures from the preserved specimen, not from your text Label only those structures which you can actually observe in the dissected specimen. (See fig. 18-20, for comparison)

On demonstration are other preserved anemonies and corals. The individual coral animals are very small but live in large colonies. Individuals secrete the calcareous structure you identify as the coral skeleton. Over the years as these build up, layer upon layer, they form large coral reefs. Examine both preserved and dried corals to see the relationship between the animals and the skeleton.

Examine the other demonstration materials and take notes. As is the case with the Scyphozoa, the Anthozoans, particularly the anemonies, do not preserve well.