LABORATORY 3: CNIDARIA and CTENOPHORA

The phylum Cnidaria includes such animals as jellyfish, sea anemones, corals and hydroids. This group was originally referred to as phylum Coelenterata, a name still seen in many textbooks. Modern taxonomists, however, prefer the phylum name Cnidaria, and use "coelenterate" only as a common name. Closely related to the phylum Cnidaria is the phylum Ctenophora containing organisms called "comb jellies." Both cnidarians and ctenophores are considered to be at the "tissue grade" of body construction. In other words, their body cells are organized into tissues and these tissues are functionally dependent on one another, but (excluding a few minor exceptions) these tissues are not organized into organs.

PHYLUM CNIDARIA

Cnidarians are radially symmetrical organisms that have one of two basic body forms: the polyp form or the medusa form. Polyps are cylindrical in shape and are generally oriented in the environment with their oral (mouth) surface directed upward and their aboral surface (opposite the mouth) attached to a substrate. Medusae are umbrella- or bell-shaped, are generally free-swimming, and are oriented in the environment with the oral surface directed downward and the aboral surface directed upward.

The life cycle of cnidarians often includes an alteration of sexual and asexual generations, a phenomenon that is referred to as metagenesis. When metagenesis occurs, the polyp is the asexual generation and the medusa is the sexual generation. A generalized life cycle occurs as follows: Medusae produce gametes which unite to form zygotes. Each zygote divides repeatedly and develops into a free-swimming larval form called a planula larva (Fig. 3.17A on p. 83 of S&S). The planula larvae eventually settle and develop into polyps. Each polyp then asexually produces medusae to complete the life cycle. This generalized life cycle is modified greatly in different groups so that either the polyp or the medusa stage may be reduced or even completely absent from the life cycle. When the medusa stage is absent from the life cycle, the polyp reproduces both asexually and sexually.

Colony formation is very common in the phylum Cnidaria, especially among polyps. When colonies form, there is a tendency for individuals within the colony to become specialized structurally and functionally. This structural and functional specialization within a colony is referred to as polymorphism. You will be observing several examples of polymorphic cnidarian colonies in the laboratory.

One particularly distinguishing feature of the phylum Cnidaria is that all members of the phylum produce structures called nematocysts. Nematocysts are produced within cells called cnidoblasts (or cnidocytes) and are discharged under the influence of mechanical, chemical or nervous stimuli. Nematocysts function in defense, prey capture,

and temporary anchorage of the body to a substrate. After a nematocyst is released, its cnidoblast cell dies. New cnidoblast cells and nematocysts are therefore continually being produced. The structure of nematocysts and cnidocytes is shown in Fig. 3.13, p.75 of S&S.

Despite the difference in shape between polyps and medusae, the internal structure of these two body forms is very similar. Each has a central coelenteron or gastrovascular (GV) cavity which functions both in digestion and in circulation of nutrients around the body. The GV cavity has a single opening to the outside--the mouth-- through which food enters the body and undigested material leaves. The body wall consists of three layers: an outer epidermis of ectodermal origin, an inner gastrodermis of endodermal origin, and a layer called mesoglea between the two. The mesoglea is gelatinous in consistency and, in some cnidarians, may have cells located in it. It is probably secreted by both the epidermis and the gastrodermis.

Most Cnidarians are carnivorous and use nematocysts to capture prey. The processes of feeding and digestion are relatively uniform throughout the Cnidaria. Captured food is passed into the mouth by the tentacles or by a tract of cilia. Digestion is partly extracellular and partly intracellular. Extracellular digestion takes place in the GV cavity as enzymes are secreted by gastrodermal cells lining the cavity. Finally, gastrodermal cells phagocytize the partially digested food particles and digestion is completed intracellularly.

Both the epidermis and the gastrodermis absorb oxygen for respiration directly from the environment or from the GV cavity. Oxygen then diffuses to any underlying cells. In the same manner waste products, such as carbon dioxide and ammonia diffuse outward from cells to the body surface. There are no specialized organs or body surfaces for gas exchange or the elimination of wastes.

The phylum Cnidaria is divided into three classes based on characteristics such as life cycle, the morphology of the GV cavity, and the presence or absence of cells in the mesoglea. The three classes are Hydrozoa, Scyphozoa and Anthozoa.

In today's laboratory we will briefly examine the diversity of cnidarian form and life styles and make observations on feeding, nematocyst function, and digestion in repesentative organisms. In addition, we will observe a representative of the closely related Ctenophora.

Class HYDROZOA

The life cycle of hydrozoans typically exhibits metagenesis, alternating between an asexual colonial polyp stage and a sexual medusa stage. However, within the class there are species that tend to reduce either the polyp or the medusa stage of the life cycle. The hydrozoan medusa usually has a circular shlef of tissue attached to the underside of the umbrella. This structure is called the velum (see Fig. 3.1B, p.61 of S&S) and it

functions in medusa locomotion. In both polyp and medusa forms within class Hydrozoa, the mesoglea is acellular and the GV cavity is a simple sac. One last distinguishing characteristic of the class is that gonads are always formed from epidermal tissue.

Genus Gonionemus

In the life cycle of Gonionemus the medusa stage is conspicuous while the polyp is very reduced. Each Gonionemus medusa produces either eggs or sperm, and the union of gametes results in the formation of a planula larva. The planula grows into a minute solitary polyp (about lmm in size) which asexually buds off other polyps. Each individual polyp then buds off a free-swimming medusa to complete the life cycle.

Examine the diagram of a Gonionemus medusa in your text (Fig. 3.6A, p.68 of S&S). Notice the velum. What do you think is the function of the velum? What is the function of a statocyst?

In some hydrozoan species closely related to Gonionemus, the medusa stage is totally absent from the life cycle. In these species the planula develops into a polyp-like larva called an actinula (Fig. 3.17, p.83 of S&S). The actinula larva develops directly into a polyp.

Genus Obelia

The life cycle of this hydrozoan has a fairly even emphasis on the polyp and medusa stages. The polyp stage of Obelia is colonial and provides a good example of polyp polymorphism. Examine a live colony of Obelia under the dissecting scope. Rfer to p.71 of S&S to identify polyp types. Two different structural and functional types of polyps can be observed in the colony. The polyps with tentacles and mouths are called gastrozooids and their function is feeding. The polyps without tentacles are called gonozooids and their function is reproduction. Can you identify medusa buds within the gonozooids? These will eventually give rise to free-swimming medusae. Often, by artificially manipulating the photoperiod of marine organisms, it is possible to observe the release of planktonic stages. Observe a portion of an Obelia colony which was kept in the dark for a couple of days and then exposed to light just before the laboratory period. Have the gonozoids released medusa? If so, examine an Obelia medusa. Why do you think this photoperiod regime would be expected to cause the release of medusa?

All of the polyps of an Obelia colony share a common GV cavity so that food ingested by the feeding polyps can be circulated to nourish the non-feeding polyps. Observe the transparent skeleton covering the polyps of the colony. This covering is secreted by the epidermis and is composed of a polysaccharide called chiton.

Genus Hydractinia

Hydractinia is a colonial hydrozoan which normally grows on snail shells occupied by a certain species of hermit crab. Observe a living colony under the dissecting microscope. You should be able to identify several different types of zooids. (Fig. 3.9 on p. 72 of S&S). The gastrozooids have tentacles and mouths used in feeding. The spiral zooids or dactylozooids are defensive in function. They are normally in a coiled position but can straighten out quickly when the colony is threatened by a predator. The small knobs at the tips of these zooids are reduced tentacles that contain batteries of nematocysts. Are the spiral zooids concentrated in any particular area on the hermit crab shell? Why might this be so? The gonozooids (reproductive polyps) bear several small swellings called medusoid buds. These medusoid buds will never release free-swimming medusae. Instead, gonads will develop within the bud and the eggs or sperm will be shed into the seawater. (Each colony bears either male or female medusoid buds, but not both.)

What aspect of Hydractinia's life history might explain why the free-swimming medusa stage has been lost in this organism? In colonial hydrozoans like Hydractinia and Obelia, into what type of zooid do the planula larvae probably develop? Why? Speculate on the relationship of Hydractinia and the hermit crabs which carry the shells they live on. What are possible advantages and disadvantages (costs and benefits) to Hydractinia and hermit crabs in this relationship? Would you describe this association as parasitism, commensalism, or mutualism? Explain.

Genus Hydra

Although they are classically studied as "typical" cnidarians, Hydra exhibit several very unique features. First of all, Hydra are freshwater organisms. This characteristic sets them apart not just from other hydrozoans but from cnidarians in general. What structural characteristic of cnidarians might account for the fact that the vast majority of cnidarians are marine?

A second characteristic that distinguishes Hydra from other hydrozoans (and also from scyphozoans) is that there is no trace of a medusa or even a medusoid bud in the Hydra life cycle. Typically, Hydra polyps are reproduce by asexual budding throughout the spring and summer, and they turn to sexual reproduction only in the fall. At that time, gonads develop directly on the polyp. When Hydra eggs are fertilized, they receive a protective covering that makes them resistant to freezing and desiccation. The eggs will develop into new polyps when environmental conditions improve the following spring.

How might this reproductive phenomenon be correlated with the fact that Hydra are freshwater organisms?

A third unusual feature of Hydra relative to other organisms in its class is that Hydra polyps are solitary rather than being colonial. The polyps do not secrete any type

of skeleton, and the individual animals are capable of considerable locomotion. Observe a living specimen of Hydra and identify the structures labelled in Fig. 3.4, p.64 of S&S.

The reason Hydra are the standard cnidarian for study in the laboratory is because they are generally available and cooperative in demonstrating a number of phenomena common to cnidarian polyps. We will use Hydra today to examine polyp functions.

Obtain a few specimens of Hydra that have been starved for 48 hours and transfer them to a syracuse watch glass containing pond water. Allow the animals to attach and relax and then note how the body column elongates and the tentacles extend. Although Hydra are usually sessile, they can glide on their base, float by means of gas bubbles secreted in the region of the basal disc, and somersault to escape predators. These movements, however, are hard to elicit in a laboratory situation. Using a probe, examine how different parts of a Hydra's body respond to a stimulus. Are all parts equally quick to respond? What does this tell you about the nervous system of Hydra?

With a pipet, transfer some Artemia larvae to the dish with the Hydra. Carefully observe the reaction of Hydra to the prey. Describe the feeding response. Note the movements of the tentacles and the reactions of the hypostome. What are the effects on the prey? How are the prey held? Remove a prey item from the tentacles with fine forceps, make a wet mount, and examine under high power of the compound microscope (use oil-immersion if necessary). Examine and describe the nematocyst types found (p.75, S&S). To examine undischarged nematocysts, remove a tentacle from a hydra with a forceps, place it on a slide, cover with a coverslip, and examine under the compound scope. How are the cnidoblasts distributed on the tentacle? To observe the discharge of nematocysts, draw 5% acetic acid under the coverslip while observing cnidoblasts.

Let us now try to determine the mechanisms responsible for feeding behavior in Hydra. Place a Hydra individual in a syracuse watchglass of water and attempt to discharge nematocysts by mechanically stimulating a tentacle with a probe or strand of hair. Do nematocysts discharge? Do you observe a feeding response in the individual? Now place isolated fresh Hydra in a watchglass and examine their reaction to introducing clam juice and reduced glutathione into the watchglass without applying mechanical stimulation. Reduced glutathione is a chemical released by injured prey. Do the nematocysts discharge? Do you see a feeding response? Construct a response-stimulus model of Hydra feeding behavior based on your observations.

Genus Physalia

Physalia, commonly called the Portuguese man-of-war, looks like a large medusa but is actually a floating hydrozoan colony (Fig. 3.10, p.72 of S&S). The float, which contains gas and keeps the colony buoyant, is so highly modified that it is questionable as to whether it is a polyp or a medusa. The float is the first member of the colony to develop and it asexually buds off the other individuals of the colony. These other individuals are modified polyps and they hang down from the lower surface of the float.

Three different types of polyps can be observed: gastrozooids, gonozooids and dactylozooids. Gastrozooids and gonozooids function as described for other hydrozoan colonies. Dactylazooids each possess an enormous, nematocyst-bearing fishing tentacle, and function in defense and in capture of food (although food items must be passed to the gastrozooids for ingestion). As in other hydrozoan colonies, the GV cavities of all members of the colony are connected. Examine the preserved Physalia available in the laboratory.

Class SCYPHOZOA

This class includes the exclusively marine “true” jellyfish and their related polyps. The medusa stage is usually large and conspicuous in the life cycle, while the polyps are small or even lacking. The GV cavity in both the polyp and medusa forms is divided into 4 gastric pouches. Medusa lack a velum and have oral arms (4 frilly extensions of tissue that hang down around the mouth). Both polyps and medusae have a cellular mesoglea, and gonads on the medusae are gastrodermal.

As a representative scyphozoan we will examine Aurelia, a common jellyfish found along the Atlantic coast of North America. The life cycle of Aurelia is given on p. 63 in S&S. The free-swimming medusae produce gametes which give rise to small polyps called scyphistomae. After a period of growth, the scyphistoma divides transversely to become a strobila that resembles a stack of discs. Each of the "discs" becomes an ephyra larva, detaches from the strobila and swims freely in the plankton. The ephyra larva will eventually grow into an adult medusa. Examine prepared slides of Aurelia planula, scyphistoma, strobila and ephyra and locate the structures indicated in Fig. 3.3.

As was true for the Hydrozoa, scyphozoan polyps (scyphistomae) may asexually produce other polyps in addition to producing medusae. New scyphistomae may be produced asexually by budding or by producing structures called podocysts. A podocyst is formed when the basal disc of the scyphistoma fragments off the parent polyp and becomes surrounded by a resistant covering of chitin. The cyst will remain dormant for a while, but will eventually give rise to a new polyp. Podocyst formation is seen in a number of different jellyfish species. Podocysts are produced seasonally and are able to survive winter conditions that would kill the polyps. Podocyst production may represent an adaptation to extreme environmental fluctuations. The production of podocysts is thus analogous to the production of resistant eggs by Hydra.

Although scyphozoan medusae resemble hydrozoan medusae in general features, they differ in that they lack a velum, they have a complex G.V. cavity, and they have compound sense organs called rhopalia around the edge of the umbrella. In addition, the angles of the mouth are elongated into four oral arms which are grooved, often frilled, and always heavily ciliated. Examine a live specimen of Aurelia. Identify the oral arms, gastric pouches, rhopalia, tentacles, and gonads. Refer to pp. 63 and 79 of S&S.

Carefully observe and describe the swimming movement. A single sadist in the class may want to demonstrate the function of the rhopalia. How would you suggest this be done?

Examine the feeding response of Aurelia by placing one in a large fingerbowl filled with seawater and then introducing Artemia larvae that have been in a suspension of carmine particles. Describe the feeding behavior. Is capture passive or active? Examine Artemia that have been captured by Aurelia and determine the types of nematocysts used. Allow the Aurelia to continue feeding during the remainder of the laboratory period and at intervals note the distribution ofingested food.

Class ANTHOZOA

This remaining class of Cnidaria contains the sea anemones and corals. Medusae are completely absent from the life cycle of anthozoans. The polyp produces gametes directly. Fertilized eggs develop into planula larvae and each planula gives rise to a new polyp. In addition to sexual reproduction, polyps may also reproduce asexually by budding or by fragmentation. Anthozoan polyps may be solitary or colonial, depending on the group. The internal structure of the anthozoan polyp is more complex than that of hydrozoan and scyphozoan polyps. The GV cavity is characteristically divided by a number of radially arranged septa or mesenteries (Fig. 3.5, p.66 of S&S). The mesoglea is always thick and contains cells and fibrous supporting material. The gonads are gastrodermal and are borne on the septa. Most Anthozoa have a bilateral or biradial symmetry superimposed on their basic radial symmetry

The Anthozoa are divided into two subclasses: 1) Zoantharia (Hexacorallia) - sea anemones and hard corals; and 2) Alcyonaria (Octocorallia) - soft corals and horny corals.

Subclass ZOANTHARIA

The tentacles and septa are often in multiples of six but never eight. The tentacles are always simple and the skeleton, if present, is an exoskeleton made of calcium carbonate.

Sea Anemones

Sea anemones are solitary polyps that usually live attached to a hard substrate. Sexual reproduction in anemones occurs as described for Anthozoa in general. Asexual methods of reproduction involve not only budding, but also fragmentation and fission. In fragmentation small bits of the basal disc are cast off as the anemone slowly creeps along. Each bit grows into a small anemone. Fission refers to the splitting of an individual longitudinally into a few large pieces which become a new polyp.

In the laboratory we will examine the anemone Metridium as a representative anthozoan. Obtain a Metridium specimen that has been allowed to settle on the bottom of a large fingerbowl. The neuromuscular system of anemones is more highly developed and localized than that of Hydra or Aurelia and can be examined in the laboratory easily. The anemone nervous system consists of a two-dimensional neuronal net without ganglionic centers. Conduction is outward in a circle from the center of stimulus, and the extent of the response depends both on the intensity and duration of the stimulus. In addition to the nerve net there are well-developed tracts containing elongate neurons which serve for rapid conduction (p.79, S&S). When your Metridium is fully expanded, stimulate a tentacle by touching the tip with a probe. What is the reaction? Is it localized? In what direction does the tentacle bend? Continue stimulating the tentacle and/or increase the stimulus strength. (Don’t overdo it!) What happens? What does this tell you about the nervous system of Metridium? Stimulate the body wall of the anemone with the same intensity that you initially stimulated the tentacle. Is there a reaction, what happens it you increase the strength of a single body wall stimulus? Explain these observations.

To examine the feeding response of Metridium rub small clam fragments in powdered carmine and present the food to the tentacles. What is the response? Does the response differ with the size of the introduced food? Are nematocysts used in the capture of the prey? Be frugal in feeding your anemone so that it will cooperate with your remaining observations. Place a small piece of stained food on the edge of the oral disc. How is the food moved toward the mouth? Do tentacles move the food into the mouth? Can you observe the activity of acontia at the base of the actinopharynx? What is their function? Try feeding your anemone a piece of wet filter paper. What happens? Next try feeding your anemone filter paper which has been wetted by absorbing fluid from clam tissue. Is there a difference in response?

Since the food you are using to feed your anemone has been "stained" with carmine particles, you should be able to trace the path of the food into the coelenteron. Try to document that the food is directed down the actinopharynx by the flagellated siphonoglyph, and inside the coelenteron by cilia on the septa. The thin septa ensure that the living tissue is nowhere very thick and diffusion paths of food and gases remain short. The actual site of digestion of food by Metridium may be determined at the end of the laboratory period. Allow the Metridium you have fed carmine stained food to stand in fresh seawater in the cold room until close to the end of the laboratory period. Then relax the fed specimens in 7.2% MgC12 and open the coelenteron by making a longitudinal incision from the base to the oral disc. Try to locate the structures diagrammed on pp. 66 and 79 of S&S. Where is the food located? Can you observe whole carmine particles in cells? Does digestion take place extracellularly or intracellularly?

Examine a cross section of your Metridium and locate the following: pharynx,siphonoglyph, complete and incomplete septa, GV cavity, retractor muscles, gastrodermis, mesoglea, and epidermis.

Hard Corals

The fundamental anatomy of the septa and the GV cavity of hard corals closely resembles that of sea anemones. The major differences involve the colonial growth pattern of most corals and the secretion of a massive calcium carbonate exoskeleton. Examine a colony of living Astrangia and identify the basic external features of the polyps.

In the most generalized coral skeleton type, the portion of the exoskeleton directly around the polyp resembles a cup with a floor and walls. Astrangia forms this type of exoskeleton. Soon after the planula larva settles and becomes a juvenile polyp, it begins forming an exoskeleton by secreting the floor of the coral cup. Almost at once the undersurface of the polyp develops radial folds which secrete radially arranged ridges called sclerosepta. These skeletal sclerosepta alternate with the tissue septa within the GV cavity of the body. At the same time a rim is formed and built up as a wall around the polyp. Study a piece of Astrangia (or other cup coral) from which the polyps have been removed.

The exoskeletons produced by other coral types are modifications of the floorwall-sclerosepta pattern seen in the cup corals. One interesting modification is seen in the mushroom coral, the only solitary coral polyp that you will study in the lab. Note that the large individual polyp that secreted this skeleton produces a floor and sclerosepta but no walls. Thus, the skeleton is relatively flat instead of being cup-shaped. A second modification is seen in the colonial rose coral and in brain corals. Here the polyps are arranged in rows, and adjacent polyps in a row are fused to one another. These polyps secrete a common wall around the whole row of polyps without producing walls between neighboring polyps. The result is the formation of a series of winding grooves on the surface of the calcareous skeletal mass. Are sclerosepta obvious in this type of coral configuration?

You should also take note of the great variety of overall shapes that entire coral colonies may acquire. Some corals produce lateral branches, while others are encrusting

or leaf-like. Still others form large compact mounds. The overall shape of the colony is determined by the pattern in which new polyps are asexually budded.

Subclass ALCYONARIA

This subclass includes sea fans (gorgonians) and sea pens (Renilla). They are always colonial. Each individual polyp of the colony has eight pinnate (feather-like) tentacles, eight complete septa and a single siphonoglyph. Note that the mesoglea of alcyonarians is relatively thick. Running through the mesoglea are numerous gastrodermal tubes that connect the GV cavities of individual polyps in the colony.

The alcyonarian skeleton is an internal one or endoskeleton which is secreted by the mesoglea. It is generally in the form of microscopic spicules of calcium carbonate. However, the horny corals -- sea whip and sea fan -- have an endoskeleton of horny protein material in addition to spicules. This horny skeleton is important in giving support to the colony. The skeleton of another alcyonarian, the organ pipe coral, is particularly interesting. This skeleton is composed of fused spicules stained red with iron salts. The spicules are secreted by the mesoglea, but they end up encasing the polyps of the colony. The skeleton is built into a series of tubes (each of which contains one polyp) strengthened by connecting transverse platforms.

In several groups of Alcyonaria there is polymorphism of individuals within a colony. This is illustrated nicely by the sea pansy and the sea pen (Fig. 3.11, p.74 of S&S). The sea pansy consists of a large primary polyp with a stem-like base that is anchored in the sand. The upper part of the primary polyp gives rise to two types of secondary polyps. Autozooids are ordinary polyps which bear tentacles, feed and reproduce. Siphonozooids lack tentacles, are small and wart-like in appearance, and occur in clusters. They do not feed, but rather serve to create a water current through the colony. What is the function of this water current?

PHYLUM CTENOPHORA

The Ctenophora are among the most beautiful of marine organisms. They are transparent, pelagic animals with bilateral symmetry superimposed on a basic radial symmetry. They are never colonial and have no sessile stage. Their most characteristic features are ctenes, which are plates of fused cilia arrangea like the teeth of a comb (see p. 172 Barnes). There are eight vertical rows of ctenes arranged at intervals around the body. The ctenes in each row beat in metachronal waves and propel the organism's mouth forward through the water. Examine the locomotion of a live specimen of Mnemiopsis (if available) under a dissecting scope in a large fingerbowl.

Like Cnidarians, all ctenophores are carnivorous and may use tentacles to capture food. Only one species of ctenophore produces nematocysts. All other ctenophores have colloblast cells, which function in food capture by sticking to prey items.

Discharged colloblast cell

Introduce some Artemia marked with carmine to the Mnemiopsis and observe prey capture. Remove a captured Artemia and examine for colloblast fragments.

Apart from the recent discovery of true nematocyst in one ctenophore species, many things indicate a close relationship between ctenophores and cnidarians. These include: 1) the properties of the cells of the mesoglea, 2) the nature and organization of the GV cavity and nerve nets, 3) the general tetraradial symmetry, and 4) the lack of organs other than sensory ones. In addition, at least one species of ctenophore has a planula-like larval stage.

TERMINOLOGY TO KNOW FOR PHYLA CNIDARIA AND CTENOPHORA

Since the cnidarian/ctenophora section includes a large number of terms, we thought it would be helpful to highlight the most important ones for you. You should be able to give a good definition or description of all the terms listed below. You should also know the function and location of any anatomical structures in the list.

PHLYUM CNIDARIA

radial symmetry biradial symmetry bilateral symmetry oral surface aboral surface epidermis gastrodermismesoglea

coelenteron (gastrovascular cavity) cnidoblast (cnidocyte) nematocyst polyp medusa metagenesis polymorphism hydroid colony gastrozooid gonozooid dactylozooid basal disc (pedal disc) manubrium velum oral arms radial canals rhopalium statocyst planula actinula scyphistoma strobila strobilation ephyra podocyst complete septum or mesentery incomplete septum or mesentery acontia septal filament retractor muscle sphincter muscle siphonoglyph pharynx (stomodeum) endoskeleton exoskeleton sclerosepta

PHYLUM CTENOPHORA

ctenes colloblast cells

HYDROZOAN LIFE CYCLES

Medusa eggs

Zygote Ciliated larva (Planula)

sperm

Life Cycle of ObeliaMetamorphosis and

settling

Budding Hydroid Colony(polyp form)

A. Tendency for reduction B. Tendency for reductionof medusa stage. of polyp stage.

Budding

Polyp Eggs and sperm Medusa Eggs and sperm

Actinula larvaPlanula larva

Life Cycle of Hydra Life Cycle of Some Close Relatives

of Gonionemus