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The Phylum Coelenterata or Cnidaria contains tentaculate animals that have primarily radial or biradial symmetry, a digestive tract with only one opening, the mouth, and possess a unique organelle, the nematocyst. These animals have cells that are differentiated into well-defined tissues derived from either two or three primary germ layers and are thus said to be of tissue grade construction, although some species may have tissues arranged to con­stitute organs.

There are two basic body plans in this phylum: the polyp and the medusa. Actually, they are essentially the same form (a sac-like body with a central mouth surrounded by tentacles at one end), but merely have emphasized opposite axes and different amounts of mesoglea (a matrix laver between the epidermis and gastrodermis). The polyp is elongated in the oral-aboral axis, thus being tubular, and has very little mesoglea. The medusa has a short oral-ahoral axis and a proportionately widened diameter, becoming cup or bell-shaped, and has a thick mesoglea layer. The polyp is sessile; the medusa is typically motile. As you study this phylum, consider how the amount of mesoglea is related or adantive to these two modes of life.

The three classes in this phylum are distinguished by characteristics of the polyp plan. The medusae when present also have distinctive features.

Class Hydrozoa

The Class Hydrozoa is composed of diploblastic coelenterates whose polyp stage has no internal septa, no stomodeum (pharynx), and no gastro­dermal cnidocytes (cells that produce nematocysts). The medusa typically possesses a velum, a circular shelf or flap projecting from the bell margin toward the mouth. Hydrozoans are hest known by the polyp state which usually forms extensive or relatively conspicuous colonies. The medusae
are either microscopic or otherwise inconspicuous, having transparent bodies.

1. Hydra - The basic polyp plan is hest exemplified by the familiar hydra. Examine under low power a prepared slide of a hydra whole mount. Note the tubular body called the body stalk. It attaches itself to the sub­strate by its aboral end, the basal disc. the opposite or oral end is a cone-shaped structure, the hypostome, which bears the mouth at its distal end and tentacles around its base. How many tentacles are there? Is this the same number as found by students next to you examining their specimens? The tentacles bear knobs formed by clumps of cnidocytes containing nematocysts. The clumps are called batteries (as in artillery terminology) which discharge their nematocysts explosively for paralyz­ing and capturing prey. Center a battery in the field of view and carefully change to high power. Try to make out the features of some nematocysts. Can you see within a cnidocyte a capsule with a coiled thread inside? If not, make a tentacle preparation from a living hydroid or hydra obtained from the supply table. Place a nolyn in a drop of the culture water on a clean slide and add a tiny drop of methylene blue and a brine shrimp larva. Place a cover slip on the preparation and gently press it down with a pencil or blunt probe handle until the tentacles and brine shrimp are squashed. Examine the tentacle area with low and high power for both undischarged and discharged nematocysts,


Draw a few of each. Refer to your text materials for details on nemato­cyst operation and variety of form.

Examine a prepared slide of the cross-section of a hydra under low and high power. The central cavity surrounded by the body wall is the gastrovascular cavity. Study the outer epidermis, the non-cellular mesoglea, and the inner gastrodermis. Which is the thickest layer? What are the chief differences between the epidermal and gastrodermal cells?
Draw an outline to show the relative thickness of the three layers in a cross-section of hydra. Fill in a section of the outline to show.
cellular detail.

Tentacles of hydroids have two forms: filiform and capitate. Filiform tentacles are like those of hydra, long and tapering, and have the batteries of nematocysts distributed all over the surface or confined along the outer edge or in circles or spirals. Capitate tentacles have the nematocyst batteries confined to a knob on the end of the tentacle. Examine a prepared slide of Pennaria. Study the two types of tentacles on the polyps.
The distribution of tentacles on the polyp oral end has various patterns.

Examine the tentacle Patterns on prepared slides of Obelia, Pennaria, and Tubularia and the demonstration of Syncoryne or Clava, lake
sketches of each pattern. The primitive condition is postulated to be
the pattern shown by Syncoryne or Clava, Based on your sketches, what do you think night be a reasonable evolutionary sequence in tentacle patterns? Number your sketches in the sequence that you postulate. Discuss this sequence with your lab instructor for comparison with the consensus among hydroid specialists.

5. Coloniality (colony formation) is the predominant trait of the polypoid form of hydroids, unlike solitary hydra. Hydra asexual reproduction gives a clue as to how coloniality was derived. Examine a prepared slide of hydra budding. How does the bud form? In hydra, the mature bud develops a mouth and tentacles and then constricts its base to pinch off and lead an independent existence. What would happen if the bud did not pinch off? Examine a Prepared slide of Obelia, The polyps form a colony by remaining attached. Can you determine the budding pattern from your specimen?

6. Once coloniality was attained in hydroids, natural selection favored the division of labor among members of a colony, resulting in morphological change, Members of hydroid colonies became modified for snecific functions. If two polyp forms evolved, the colonies are dimorphic.

Three or more polyp forms associated together are said to he polymorphic.

Hydroids have evolved the following basic polyp forms:
gastrozooid - prey capture and feeding form, generally least modified from basic polyp plan; sometimes called hydranth,
dactylozooid - defensive polyp; mouth, tentacles, and gastrovascular cavity reduced or absent; form may resemble a sinnle tentacle; protects and/or assists gastrozooid in prey capture; sometimes called nematophore.


gonozooid.- stalk-like form that buds medusoid or morphologically equivalent stages; sometimes called blastostyle,
pneumatophore - gas-filled sac or disc-shaped polyp acting as a float. This form may also develop as a medusa).

Coloniality takes three characteristic forms in hydrozoans: a) horizontal or adnate, in which the colony members generally bud off each new member from a common colony base (hydrorhiza) growing over the substrate; h) vertical or erect, in which the colony members generally bud off new members from a common colony stalk (hydrocaulus) or from the sides of the erect polyps; and c) floating, in which one individual functions as a float (pneumatophore) and buds other colony members which are suspended from it.
The following organisms (7-12) should be studied and compared for colony form, types, form and arrangements of polymorphic polyps, and observations on colony integration. lake appropriate sketches of the colonies and individual polyps to illustrate these features and label fully.

7. Obelia - Examine a prepared slide of Obelia, whole mount, under low
power of the compound microscope. The colony has, a non-living, chitinous covering secreted by the epidermis called the perisarc. What are its functions? The colonial tissues making up the body wall of the hydrocaulus is known as a coenosarc ("common flesh"). Note the form
and arrangement of the gastrozooids or hydranths. How are the gonozooids or blastostyles (club-shaped polyps that bud medusa) positioned with respect to the gastrozooids? Note that the perisarc forms a cup, the hydrotheca, around each gastrozooid. It also forms a covering, the gonotheca, enclosing the gonozooid. A gonozooid and its gonotheca make up a structure called a gonangium. How does a gonozooid receive nourish­ment. How is it protected?

8. Hydractinia or Podocoryne - Obtain a living specimen of either of these genera from the supply table. (Use preserved material if living colonies are not available). Examine them under the dissecting microscope.
Avoid getting seawater on the microscope. These hydroids have a symbiotic relationship with hermit crabs, surviving only on shells occunied by the crabs. Note the colony form and the form and distribution of gastro­zooids, dactylozooids, and gonozooids. Especially long dactylozooids may be found along the lip of the shell aperture. Is a perisarc present? Are there structures Present to protect the colony from mechanical damage? Gonozooids may be recognized by the presence of medusa buds. Do the gonozooids have tentacles?

Feed the living hydroids some live brine shrimp nauplii obtained from the supply table. Do this while observing the colony under the dissecting microscope. Squirt the nauplii from the pipette so that they are likely to bump into the polyps. latch carefully prey capture
and ingestion. How does the size of the prey compare with the size of the gastrozooids?

9. Plumularia - lake a wet mount preparation of a piece of preserved
Plumularia and examine under low power of the compound microscope.
Note the colony form and the form and distribution of the gastrozooids.


Associated with each gastrozooid are three dactylozooids (nematonhores): an elongate, tubular one alone the median frontal side of the hydrotheca, and two short, tubular or cup-shaped ones located one each side of the disto-lateral borders of the hydrotheca. These descriptions actually apply to the theca of each dactylozooid. The dactylozooids themselves are club-shaped individuals terminating in a knob of cnidocytes and/or adhesive cells. A short tentacle-like extension containing adhesive cells may be present on some dactylozooids. Examine the cnidocytes with high power. What is the shape of the nematocysts? See if you can
find other dactylozooids along the hydrocaulus. How does the gastro‑
zooid-dactylozooid relationship in Plumularia differ from, that observed in Hydractinia or Podocoryne? which would you speculate has the more specialized condition? Why?

Millepora - Hydrozoan corals are noted for their virulent stinging
capacity, hence their common name of "fire corals". Examine the
demonstration of Millepora. Look at the surface of the calcareous
skeleton (equivalent to the perisarc of other hydrozoans) under the dissecting microscope. In addition to the tiny pores in the frothy
looking skeleton, notice that there are two sizes of larger pores. The largest Pores are called qastropores and each is occupied in life by a gastrozooid. The smaller pores are called dactylopores and from each in life would protrude a tall dactylozooid bearing numerous capitate tentacles. Which are more numerous, gastropores or dactylopores? You should be able to discern a pattern in the distribution of these two pore types. How are the dactylopores arranged with respect to the gastropores? Which type of colony growth plan does Millepora have?

Physalia - Examine the demonstration of Physalia, the Portugese man-of-war
Although it may appear to the contrary, Physalia is a colony. The founding individual of the colony produces the float or pneumatophore, from which is budded the other Polyp, types. What is the composition of the gas in the pneumatophore? The gas-producing gland is located on the floor of the pneumatophore and usually is not visible in Preserved specimens. Note that the pneumatophore has an erectile crest that permits some maneuvering in the wind. The clusters of nolyns are called cormidia. A cormidium is usually composed of at least one each of a gastrozooid, a dactylozooid, and a gonozooid. In Physalia, it is difficult to distinguish the separate cormidia; however, the polyp types are quite evident. The dactylozooids are of two sizes: the long "fishing tentacles" and shorter ones clustered with the gastrozooids. Each dactylozooid constitutes a polyp with a single tentacle loaded with batteries of nematocysts. How are the batteries arranged on the dactylo­zooid tentacle?

Examine a prepared slide of a Physalia tentacle section under low power of a compound microscope. Find a section that shows an elongate portion bordered by conspicuous parallel ribs and having at one end some ruffled or circular structures lacking the ribs. Relate these two portions to parts of an intact dactylozooid of the demonstra­tion specimen. Which section Parts have nematocysts? The parallel ribs of nesoglea support the epitheliomuscular cells whose contractile portions can be seen as the feathery border of the ribs. Is the tentacle solid or hollow? Study some of the nematocysts under high power. How do they compare in size to the other nematocysts you examined earlier?


Examine the preserved Physalia again. Gastrozooids occur in great abundance as smooth-walled tubes-of varying lengths hanging in dense clusters with the shorter dactylozooids. See if you can detect the Presence of prey inside some of the larger ones. Larger prey is digested cooperatively by several gastrozooids applying wide open mouths against the prey body so that gastrodermal cells are in contact with the food.

Gonozooids, when mature, appear as a dense mop of fine, somewhat flattened tubes bearing clusters of stalked spheres, the gonophores. The gonophores, which will produce the gametes, are not polyps but medusoid forms®

12. Velella and Porpita - Examine demonstration specimens of Velella and Porpita. The pneumatophore is a compartmentalized disc from which is suspended a large central gastrozooid encircled by an inner ring of gonozooids and an outer ring of dactylozooids. How do the pneumatophore and dactylozooids differ between these two genera?

The second basic body plan is the medusa. The primary function of this stage is sexual reproduction; there ore, it is given an additional name, the gonophore ("gonad bearer").

13. Gonionemus - Place a preserved specimen of Gonionemus in a finger bowl
or dissecting tray, and add enough water to just cover the specimen.
Examine the specimen under the dissecting microscope. Gonionemus has a "generalized" hydromedusan shape. Locate the following structures:

exumbrella - aboral, convex surface.
subumbrella - oral, concave surface.
manubrium (= hypostome) - tubular structure suspended from center of subumbrellar surface.
mouth on end of manubrium.

stomach - central pocket in mesoglea formed by the junction of the manubrium and the
radial canals - four tubes of gastrodermis that lead from stomach to the bell margin where they join a
ring canal - passing around the margin
gonads curtain-like folds of tissue formed in the subumbrellar epidermis under the radial canals.

Turn the specimen subumbrellar surface up. Locate a circular flap of tissue along the inner margin of the bell and projecting toward the manubrium. This structure, the velum, is characteristic of the medusa of the Hydrozoa. It is used in locomotion and possesses striated muscle


fibers for rapid contraction. How do medusae swim? What role does the mesoglea play in locomotion? Examine the tentacular bulbs at the base of tentacles. Internally, these bulbs have thickened gastrodermis lining their cavities. These are important digestive sites along with the manubrium and stomach. The epidermis of the tentacular bulbs is an important nematocyst generating site. Note the arrangement of nematocyst batteries on the tentacles. An adhesive cup may be evident on many of the tentacles. Medusae also have sense organs - structures lacking in polyps. Look carefully along the bell margin between the tentacle bases for tiny capsules, the statocysts. These are important in body orientation. How does a statocyst work? Medusae of other genera may have ocelli, photo­receptor organs, along the bell margin, but these are lacking in Gonionemus.

Examine a prepared slide of Gonionemus, vertical (radial) section, under the dissecting microscope. Use a white background. Identify manubrium, velum, subumbrellar surface, ring canals and tentacles. Now place the slide under a compound microscope and examine with low power. Is the mesoglea cellular? Focus on the junction of the velum with the bell margin. Locate the ring canal and its gastrodermal lining. Note the thickened subumbrellar circular muscle band seen in section on the outer wall of the ring canal. It is continuous with the inner (subumbrellar side) velar muscle layer which also consists of striated muscle fibers.

A layer of radial smooth muscle fibers overlies the subumbrellar circular muscle band. Using high newer, focus on the base of the velum and follow its mesolamella (thin meson-lea separating the two epithelial muscle layers) into the hell margin. On either side of this mesolamella are two areas of large vacuolated cells, the outer and inner nerve rings. The inner ring is not as conspicuous as the outer one. The darker stain­ing cells overlying the outer nerve ring are cnidoblasts (forming nematocysts) and sensory cells.
Medusae are dioecious, generally releasing eggs or sperm into the seawater where fertilization takes place. What are the advantages of having the medusa as the sexual stage?

14. Obelia - Examine a prepared slide of Obelia, hydroid colony, under low power of the compound microscope. Find a gonozooid or blastostyle and observe the medusa buds of various ages forming as evaginations of the gonozooid body wall. Where are the oldest buds? When mature, the
medusa-gonozooid connection constricts and the medusa is liberated through the top of the gonotheca.

Examine a prepared slide of Obelia medusa with low power of the compound microscope. Locate manubrium, mouth, gonads, radial canals, ring canal, tentacles, tentacle bulbs, exunbrella and, subumbrella. Change to high power and examine the tentacle bases for statocysts. There will he two of these per sector formed by the radial canals. They will apnear as clear vesicles suspended from the tentacle bases. Statoliths, the concretions which stimulate the sensory receptors of the statocysts, usually are dissolved out in the slide preparation. The velum in Obelia is rudimentary or missing (an embarassing fact to biology professors).

Draw and label fully an Obelia medusa.


Just as with the polyp stages of hydrozoans, medusae have undergone modifications. With the exception of the Order Siphonophora (Portugese man-of-war and relatives), medusa form changes have not been associated with functional changes. The trend has been associated with a reduction or loss of a free-swimming existence to complete degeneration of form except for gonadal tissues. The following series will exemplify this trend. Gonionemus and other hydrozoan medusae on demonstration are good examples of long-lived swimming medusae. Obelia, with its loss of the velum has become a less efficient and probably shorter lived swimmer.

Pennaria - Obtain a living snecimen or a Prepared slide of Pennaria. Examine the specimen or slide with the appronriate microscope and look for medusae budding from the base of the hypostome of a gastrozooid (hydranth). Mote that even very large medusa buds lack tentacles. What does this tell you about the life span of the free-swimming medusa? Gonads develop in the epidermis of the manuhrium. The mature ovaries of the female medusa will almost completely fill the subumbrellar cavity. Testes of males are not so bulky.

Draw a Pennaria medusa.

Campanularia - Examine a prepared slide of Campanularia, whole mount, under the compound microscope. This hydroid looks very much like Obelia. The medusa buds form as evaginations of the gonozooid (blastostyle) wall but never develop to more than spherical objects that house the gonads. It is common practice to call such modified medusae gonophores. You may be able to determine the sex of your specimen by observing the gonadal tissue under high power. The sperm are liberated into the gonangium chamber and escape by way of the distal end. Eggs are retained in the gonophore where they are fertilized and develop into the planula larvae which then leave the gonangium.

Tubularia - Examine a prepared slide of Tubularia, whole mount, under the compound microscope. Gonophores appear on a branched stalk arising from the hypostome, just above the base of the proximal whorl of tentacles. The branching stalk may he considered a gonozooid. As in Campanularia, the eggs are brooded in the presumed subumbrellar chamber of the female gonophore. They are retained until the planula larva has developed into a polypoid larva known as an actinula. An actinula is on demonstration for your examination.

Eudendrium - Examine the prepared slides of male and female Eudendrium or obtain living colonies in seawater for study. Gonophores of live female colonies appear orange; male gonophores are white. The gonophores develop on gonozooids that are essentially transformed gastrozooids. The gonophores are dimorphic. The male gonophores develop in linear groups of about five, these groups being clustered in a bouquet at the end of the gonozooid. Female gonophores are highly modified, being reduced to a curling stalk in whose side an egg develops. Four or more gonophores develop in a cluster from the gonozooid. Highly modified gonophores such as these are commonly called sporosacs; their central core, the spadix, produces the gametes in its walls.

Eudendrium female sporosacs release planula larvae.
Examine the demonstration of a planula. The planula is a ciliated stereogastrula. How many cell layers are present?


19. Hydra - Examine the demonstrations of male and female hydras. No gono­phores are present, the gametes develop in the epidermis of the polyp. The degeneration trend of the medusa is complete. Or could it not have been the other way around, with hydra polyps being the ancestral condi­tion and sexual medusae the ultimate in evolution! Does the fact that hydra has no planula larva affect your thinking about which evolutionary direction is most likely? What other characteristics of hydra affect the interpretation?

Look up the classification of all genera in this exercise.

(This material comes from ZOO 6207, "Advanced Invertebrate Zoology", at the University of Florida, Spring Semester 1982, class taught by Dr. Frank Maturo. The text was scanned and OCR'd by Wesley R. Elsberry in 2006. Typographic errors are likely due to the scanning process.)