Chapter 13 - The Radiate Animals: Phylum Cnidaria, Phylum Ctenophora

Gastric Pouches

Four rounded gastric pouches extend from the digestive cavity.

Definite Tissues

Cnidarians are the lowest animals with definite tissues.

Radial Symmetry

Cnidarians have a radial symmetry around an oral aboral axis, no definite head.

Oral arms

Four tapering oral arms extend from the manubrium. The oral arms are used for transporting food to the mouth. Aurelia feeds on plankton and small organic particles, which it traps on mucus on the ciliated subumbrellar surface. The food is then transported by the beating of the cilia to the bell margin. There it is scraped off by the oral arms and carried by way of ciliated grooves on the oral arms to the mouth.

Gullet

From the mouth, the food passes through a short gullet into the enter on of the gastrovascular cavity.

Nerve Net Hydra

Hydra possesses the type of nervous system known as a nerve net. This is the first and simplest neural mechanism to be seen among animals. A nerve net is a system of interconnected nerve cells. A stimulus arises in a particular part of the body spreads throughout the system. The nerve net coordinates the movements of the body and tentacles.

Budding

Hydra reproduces asexually by budding, a process in which a new hydra forms as a projection about midway on the body wall. This projection grows larger, acquires blunt tentacles and a mouth distally, and eventually constricts off, forming a new individual hydra.

Gullet or Stomodeum

The mouth leads to the gullet or stomodeoum, which is a flat tube.

Cnidae

The name of this phylum is derived from the presence of stinging structures called cnidae (formerly called nematocysts). They are used for food-getting and defense. They have a variety of functions, including prey capture, defense, locomotion, and attachment. They are oval capsules containing a coiled tubular "thread" or filament that may bear tiny barbs, or spines. The top of the capsule is covered by an opculum.

Nerve Net

The nervous system is a nerve net. This is the first and simplest neural mechanism to be seen among animals. Cnidarians are the simplest animals to have sense organs. They lack cephalization, a centralized nervous system, and discrete respiratory, circulatory, and excretory organs.

Pedal Disk

The opposite end is the base or pedal end, which is attached to something solid on the sea floor

Scyphistoma

The planula develops into a minute polyp known as this. Transverse fission (strobiliation) of the scyphistoma forms a number of ephyrae that resembles a stack of plates. The ephyrae split off from the scyphistoma, and escape into the water where they develop into adult jellyfish.

Colonial Hydroids Polyp

The polyp of Obelia consists of a branching colony. Arising from the body are two types of polyps. The feeding polyp or hydranth is a hydra like, with 20 or more tentacles. The reproductive polyp is called the gonangium.

Gastric Filaments

Small tentacle like structures, called gastric filaments, protrude from the lining of the gastric pouches. The gastric filaments bear nematocysts and are also the source of some digestive enzymes. Preliminary digestion occurs in the gastric vacuity and pouches. The distribution of partially digested food is accomplished by a system of canals that arise in the gastric pouches.

Triploblastic

Some are triploblastic, processing a third mesodermal layer, which is derived embryologically from the ectoderm.

The body wall of Cnidarians is composed of two cellular layers...

An outer epidermis composed of epithelium that covers the surface, an inner gastrodermis, composed of epithelium that lines the gastrovascular cavity and in between these two layers, a gelatinous middle layer called mesoglea.

Bioluminescence

Ctenophores have the ability to produce light (bioluminescence), which is conspicuous at night. The light emanates from cells below the ciliated bands (comb rows) and as the cilia in the comb plates beat, it gives the animal a shimmering appearance.

Ostia

In the septa are openings or ostia, through which water can pass between the internal compartments.

Biradial Symmetry

One plane is a longitudinal plane that passes through the center and that divides the body midway between the tentacles, so that one half contains the left tentacle and the other the right. The other planes is at a right angle to this and passes through the middle of the tentacles. dividing each, and the body as a whole into equivalent halves. This type of symmetry is known and biradial symmetry.

Phylum Ctenophora

is made up of the comb jellies. These are marine animals that resemble jellyfish. Nematocysts are lacking. The body is spherical or egg-shaped with the mouth at one end. Ciliated plates, known as combs are arranged radially around the body. The beating of the cilia is responsible for the locomotion of the body. Extending from the region of the mouth is a pair of tentacles. if it were not for these tentacles, the animal would have radial symmetry. However, because of the paired tentacles, only one or two planes can divide the body into symmetrical halves.

The phylum includes four classes...

1) Class Hydrozoa - includes Hydra, Obelia, Portugese man-of-war. 2) Class Scyphozoa - includes the true jellyfish. Example: Aurelia 3) Class Cubozoa - Cube jellyfishes 4) Class Anthozoa - the sea anemones and corals. Example: Metridium

Pedal Laceration

A small piece breaks off at the base of the anemone, moves away and develops into a new individual.

Polyp

A tubular body having one end closed and attached and the other end with a central mouth surrounded by soft tentacles. The tubular body form with a mouth at the oral end surrounded by tentacles. The aboral end is usually attached to the substratum by a pedal disc or other device. Polyps are chiefly attached, sessile, benthic animals, which reproduce asexually.

Siphonglyph

Along one or both sides of the gullet is a smooth ciliated furrow, the siphonoglyph. The siphonoglyph creates a current of water that is drawn down one side of the gullet, circulates around inside the gastrovascular cavity, and exits up the other side of the gullet. The gullet opens into the gastrovascular cavity. Internally, the body is divided into compartments by septa. Complete septa extend from the body wall to the pharynx. There are six pairs of complete septa. Between these are incomplete septa attached to the body wall but extending only partway into the cavity.

Septal Filament

Along the inner margin of each septum is a convoluted septal filament, continued below as a threadlike aconite. Both parts bear nematocysts and gland cells. The aconite can be protruded through pores in the body wall or through the mouth, to aid in subduing prey or for locomotion.

Hypnotoxin

As the filament stings the prey, a poison known as hypnotoxin is injected that paralyzes the prey.

The body of Hydra is a cylindrical polyp with the lower end closed to form a...

Basal Disk or "foot" which is used for attaching to objects and also for locomotion The opposite end contains the mouth, which is located on a conical hypos tome. The mouth is encircles by 6-10 hollow tentacles. Scattered over the body are batteries of stinging cells known as cnidocytes. The cnidocytes contain stinging organelles known as nematocysts that are used for defense and food getting. They are particularly numerous on the tentacles. The mouth leads into a gastrovascular cavity. The gastrovascular cavity is continuous with cavities that extend into the tentacles. The side of the body may bear lateral buds that give rise to the new individuals by asexual reproduction; at times it bears other projections, the ovaries or testes, concerned with sexual reproduction.

External Fertilization

Because the fusion of egg and sperm takes place outside the body in the seawater, this type of fertilization is called external fertilization. There, each zygote develops a central cavity, becomes attached to the substrate, and proceeds to form a new colony by asexual budding.

Diploblastic

Embryo with two germ layers (diploblastic), ectoderm and endoderm.

Epitheliomuscular cells

In the epidermis, these cells are referred to as epitheliomuscular cells, and in the gastrodermis are called nutritive-muscular cells. The myonemes contain contractile fibers.

Anemones reproduce asexually by...

Longitudinal Fission & Pedal Laceration

Hydra Sexual Reproduction

Most species of hydra are dioecious, any one individual producing only male and female sex organs and cells; a few species are monoecious. The gonads usually appear in the autumn. The testes produce sperm and the ovaries produce eggs. The sperm escape through a pore in the testis to swim in the water. The egg forms under the epidermis. The mature ovum squeezes through a small opening on its surface. Fertilization then occurs with the entrance of a sperm head.

Myoepithelial

Movement of the body of Hydra is due to the action of cells known as this.

Cnidoblasts

Nematocysts are produced by cells known as cnidoblasts. Once the cnida is fully formed, the cell is properly called a cnidocyte

Colonial Hydroids Obelia

Obelia is a colonial hydroid. During its life cycle, Obelia undergoes an alteration of generations. It alternates between an asexual polyp, and a sexual medusa. Such an alteration of sexual and asexual generations is termed metagenesis.

Oral Disk

On the upper end is a flat oral disk, which contains the mouth, surrounded by many short hollow tentacles.

Cnidarians

Radially symmetrical, diploblastic, or triploblastic Metazoa organized at a tissue level. Cnidarians are characterized by stinging structures known as cnidae and alteration of polyp and medusa stages in the life cycle.

Planula Larva

Reproduction commonly by alteration of generations (metagenesis), with asexual budding in the attached (polyp) stage and with sexual reproduction by gametes in the free (medusa) stage. Sexual forms are monoecious or dioecious. Zygote develops into a ciliated planula larva. The larva swims in the water for a while, then attaches to the bottom and develops into a mature anemone.

Diffusion

Respiration and excretion occur by diffusion through the body wall.

Longitudinal Fission

The animal splits into two along the oral-aboral axis. In budding, a small growth develops along the side of the animal. It soon comes to resemble a miniature anemone. Eventually it constricts off and either becomes a new separated individual or adds to the size of the colony.

Oral-aboral

The body parts are arranged in radial and/or concentric positions around the oral-aboral axis that extends from mouth to base. A vertical plane, that is, a plane that passes through the oral-aboral axis and the center of the animal, will divide the animal into corresponding halves. The body parts such as the tentacles simultaneously face outward in all directions.

Cnidocil

The cnidocyte is equipped with a triggering divide on its surface called a cnidocil that when contacted will cause the filament within the nematocyst to be shot out.

Subumbrella

The concave lower (oral) surface is the subumbrella. The margin of the bell is fringed by a row of tentacles.

Exumbrella

The convex upper (aboral) surface is called the exumbrella.

Gastrovascular Cavity

The digestive cavity is a saclike gastrovascular cavity, also known as an incomplete digestive tract. This type of digestive tract has single opening; there is a mouth but no anus.

Medusa

The free-swimming medusa, a gelatinous body of umbrella shape, margined with tentacles, and having the mouth on a central projection of the concave surface. The mouth is usually centered on the concave side, and tentacles extend from the rim of the umbrella.

Colonial Hydroids Medusa

The medusa is a minute jellyfish, shaped like an umbrella, and rimmed with tentacles. The body consists of an outer epidermis, an inner gastrodermis, with a gelatinous mesoglea in between. A shelflike velum extends inward from the margin of the bell partially closing the open side of the bell. it is used in swimming.

Manubrium

The mouth is located on the concave lower surface, in the center of a projection known as the manubrium.

Blastostyle

The reproductive polyp, the gonangium, contains a central axis known as this. A series of buds develop on the central axis and eventually constrict off to become free-swimming medusae. The medusae escape from an opening in the end of the gonangium and can swim freely, thus providing for the dispersal or spread of species. The medusa of Obelia have separate sexes; ovaries are formed in some and testes in others. Sexually mature medusae release their eggs or sperm into the water, where fertilization occurs.

Rhopalia

The sense organs, known as rhopalia, are positioned on the margin of the bell in eight indentations between two small lappets. Each indentation bears an eyespot that detects light, a statocyst for equilibrium, and sensory pits probably containing chemoreceptors.

Lappets

The umbrellar margin is often scalloped into rounded lobes called lappets, which provide flexion points during bell contraction.

Polyp and Medusa

There are two body types found in members of this group.

Myonemes

These columnar cells bear flattened, contractile, basal extensions are called this.

Radial Canals

This system is made up of many radial canals that lead from the gastric pouches, and extend throughout the mesoglea to a ring canal that encircles the bell margin. partially digested food is propelled and distributed through the canal system by the action of flagellated gastrodermal cells. Digestion is completed intracellularly within nutritive muscular cells. Nutrients enter the cells by phagocytois and pinocytosis and are broken down within their vacuoles.

Metagenesis

When both phases are present in a species' life cycle, it is said to undergo an alteration of generations, or as it is sometimes called, metagenesis.

Scyphozoa

includes the true jellyfish, such as Aurelia. The true jellyfish lack a velum, which is a shelf that projects inward from the edge of the bell of Gonionomous and other hydroid medusa. In the Scyphozoa, the medusa is the major stage in the life cycle. The polyp is either minute or lacking.

Gonads

or reproductive organs, are located on the margins of the septa.