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Biology LibreTexts

Introduction to Animals, Sponges, Cnidarians

Characteristics of Animals

Animals are multicellular

Except for sponges, animal cells are arranged into tissues. Tissues are necessary to produce organs and organ systems.

Tissues, organs, and organ systems enabled the evolution of large, multicellular bodies.

Animal cells lack cell walls

The cells are held together by protein structures called junctions that extend from one cell to another. An abundance of extracellular proteins also support the cells.

A skeleton supports the tissues of large animals.

Animals have a period of embryonic development

During embryonic development, cells become specialized and tissues form. The growth of tissues, organs, and organ systems therefore requires a period of embryonic development.

Animals are heterotrophs

Heterotrophs consume their organic food. Except for sponges, they ingest food and digest it in a central cavity.

Recall that fungi are also heterotrophs but fungi do not ingest their food. Fungi secrete enzymes into their environment and absorb broken down organic food products.

Animals are motile

Heterotrophy often requires motility to capture prey. Animals have motility during at least some part of their life cycle.

Animals have nervous and muscle tissue

Muscle tissue allows animals to move. Nervous tissue allows rapid intercellular communication and enables coordinated movement and response to stimuli.

Animals are diploid (diplontic life cycle)

Their gametes are heterogametes (different sizes); eggs are larger than sperm. 

The sperm are flagellated.

Gametes are produced by meiosis

The development of some animals includes one or more larval stages. Larvae refers to immature individuals of species in which the body form of the immature individuals (the larvae) is very different than the body form of the adult. Because larvae and adults have different forms, they often eat different food and may live in different habitats. Larvae are transformed into adults by a developmental process called metamorphosis.

A typical animal life cycle is shown below.


Types of Symmetry

Radial Symmetry

The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends from the center. The animal can be cut along the axis in more than one plane to produce identical halves. Animals that exhibit radial symmetry tend to be sessile (immobile). Radial symmetry allows them to reach out in all directions.

Bilateral Symmetry

Only one cut along the longitudinal axis will produce identical halves of a bilaterally symmetrical animal. Bilateral symmetry is best for motile animals.


Asymmetrical animals have no pattern of symmetry. The simplest animals (sponges) are asymmetrical.

Evolution of Symmetry

Sponges lack symmetry, and Cnidarians exhibit radial symmetry. The remainder of the phyla listed below have bilateral symmetry.

Body Plans

Embryonic Development

A fertilized animal egg divides to produce a solid ball of cells. Then, cell migration results in a hollow ball called a blastula.

Some cells of the blastula migrate inward producing a gastrula. The opening is the blastopore. The tube produced by this process will become the gut (digestive tract) of the mature animal. In species that have a separate mouth and anus, the tube will eventually extend through the length of the embryo and fuse with the opposite side. One opening will become the mouth, the other will become the anus.

In the diagram below, a circle is used to represent a blastula.

Embryonic Germ Layers

The three layers of tissues that become established during early embryonic development are called germ layers. They give rise to the body tissues. These layers are ectoderm, mesoderm, and endoderm.

The diagram below shows a cross section of an animal embryo.

The ectoderm forms from the outer layer of cells. It gives rise to the skin and nervous system.

The cells that formed the tube-like structure in the gastrula (see the diagram above) are endoderm. These cells will form the lining of the gut and the organs derived from the gut.

Mesoderm forms between the ectoderm and endoderm. It becomes the muscles, connective tissues, skeleton, kidneys, circulatory and reproductive organs

Body Cavity

The body cavity is a fluid-filled space that separates the gut and internal organs from the rest of the body.

It isolates the internal organs from body-wall movements.

It also bathes the internal organs in a liquid through which nutrients and wastes can diffuse.

Arrangement of Ectoderm, Mesoderm, and Endoderm

An acoelomate animal does not have a body cavity.

A pseudocoelomate animal has a body cavity (called a pseudocoelom) located between endoderm and mesoderm.

The body cavity of a coelomate animal (called a coelom) is located within the mesoderm.

The mesentery holds the gut in place.

The diagram below shows the body plans for nine major phyla of animals.


The gut is the digestive tract. It enables the animal to digest food outside of the cells (extracellular digestion). In animals without a digestive tract, food items are brought into the cell for digestion (intracellular digestion).

A sac-like gut has one opening. Food enters and leaves through the same opening.

A complete gut has two openings, a mouth and an anus. It is sometimes referred to as a tube-within-a-tube.

This type of gut allows for the specialization of parts along the tube. For example, part of the gut can become specialized for food storage, other parts can become specialized for secreting digestive enzymes and other parts for absorbing nutrients.


Large, Active Animals

Small animals do not require any special means to distribute nutrients and gasses or to collect wastes because every cell in the body is near a source of food. If the cells are in contact with the external environment, it is not necessary to collect wastes for removal. As evolution proceeded toward larger forms however, special structures evolved to facilitate these processes.

Circulatory System

Larger animals require a circulatory system to transport nutrients, gasses, and wastes. Fluid within the body cavity can act like a circulatory system and distribute nutrients and gasses. 

In an open circulatory system, blood leaves the blood and flows freely within the tissues. This system is not very efficient because there is no blood pressure to move blood rapidly through the tissues. The oval line in the diagram below represents an animals body.

Blood does not leave the blood vessels in a closed circulatory system. In this type of system, the heart can pump blood through the tissues rapidly.

Gas Exchange (Respiration)

All animals need to take in O2 and eliminate CO2. Lungs are membranous structures designed for gas exchange in a terrestrial environment. Gills are designed for gas exchange in an aquatic environment.

Oxygen must be dissolved in water before it can be absorbed by the respiratory structures (gills,lungs, etc.). Therefore, the respiratory surfaces of animals (must always be moist. Oxygen is absorbed from the water coating the surface.

Respiratory (Oxygen-Carrying) Pigments

The oxygen-carrying capacity of blood can be increased if the blood contains molecules that are capable of binding to oxygen. These molecules are referred to as pigments because they are colored. For example, hemoglobin is a red, iron-containing pigment found in the blood of vertebrates. Hemocyanin is a bluish-colored copper-containing pigment found in many mollusks and arthropods.

Nervous System

A nervous system receives information from sensory receptors and it is responsible for stimulating the muscles and glands. Muscle movement requires stimulation by the nervous system.

Animals that are attached (sessile) do not need sophisticated sense organs and consequently do not need an elaborate nervous system to service the sense organs. Animals that move, however, need sense organs located on the anterior end (head region) so that they can sense the environment that they move into. 

A concentration of sense organs requires a concentration of nervous tissue to receive the information and decide what to do with it. The brain is a concentration of nervous tissue near the sense organs.

Cephalization refers to the degree of development of the brain. The evolutionary trend is more elaborate and sensitive sense organs and increased cephalization.

Some Anatomical Terms

You will not be required to learn these terms for an exam, however, this list can serve as a reference.

  • Ventral - the underside
  • Dorsal - the back of the animal; the side opposite the ventral side. The vertebral column of vertebrates is no the dorsal side of the animal.
  • Lateral - toward the side
  • Median - toward the middle
  • Anterior - the head end
  • Posterior - the end opposite the head end
  • Caudal - toward the tail
  • Cranial - toward the head
  • Longitudinal - along a line from the head to the tail
  • Transverse - along a line that is 90 degrees to the longitudinal axis (see above)
  • Superficial - shallow
  • Pectoral - toward the forelimbs
  • Pelvic - toward the rear limbs
  • Distal - far from
  • Proximal - near

Sponges (Phyla: Calcarea and Silicea)

Sponges are multicellular but are thought to have evolved from unicellular protists.

Sponges are sessile (immobile) filter feeders.

Click on the photographs to view enlargements. Click "Back" to return here.

Structure of Sponges

Although sponges have cells with specialized functions they do not have tissues. There is no endoderm, mesoderm, or ectoderm.

The cells of a sponge are arranged into an inner and an outer layer with amoeboid cells crawling within a gelatinous layer that separates the two. This middle layer is the mesohyl.

Collar Cells (choanocytes)

The inner layer is composed of flagellated collar cells (choanocytes). The flagella beat to move water in through the pores and out the osculum. Food is trapped by the collar cells and is digested within the cell (intracellular digestion) or is passed to amoeboid cells for digestion.

Choanocytes (collar cells) are similar to protists called choanoflagellates. These unicellular protists are thought to be the ancestors of sponges.

Amoeboid Cells

Food particles trapped by collar cells are passed to amoeboid cells for digestion and circulation.

Amoeboid cells secrete hard mineral needle-like structures called spicules and skeletal fibers made of a protein called spongin. Spongin gives a commercial sponge its elastic characteristics.

The photograph below is a cross section of a sponge (Grantia) magnified 400 times. Notice the outer epidermal cells, pores, and choanocytes.


Most sponges exhibit sequential hermaphroditism; they function as one sex for a period of time and then change to the other sex. This prevents self-fertilization.

Gametes are produced by amoebocytes or choanocytes. Sperm are released into the water; eggs are fertilized within the mesohyl.

Although the adult is stationary, the zygote develops into a ciliated larva and swims to a new location.

Asexual reproduction is by fragmentation and budding.

Skeletal Material

Two types of skeletal material support the cells of sponges. Spicules are needle-like structures composed of either calcium carbonate or silica and offer support and protection. Spongin is a fibrous protein that provides support and elasticity. Commercial sponges are composed of spongin.


Sponges may resemble a colony of protists more than multicellular animals; they have no true tissues.

Cnidarians (Phylum Cnidaria)

Some examples of Cnidarians are hydra, jellyfishes, corals, sea anemones, and Portuguese man-of-wars.

The photograph below is a hydra (40X).


Radial symmetry

Cnidarians have two tissue layers. The outer layer is the epidermis. It is formed from ectoderm. The inner layer, the gastrodermis, secretes digestive juices into the inner space called the gastrovascular cavity.  The gastrodermis is formed from endoderm.

Below: Hydra cross section (X100). Note the two cell layers in both photographs below.

Below: Hydra longitudinal section (X100).

Cnidarians do not have mesoderm and therefore do not have organs.

A nonliving gelatinous material called mesoglea separates the two tissue layers. A nerve net is located between the epidermis and mesoglea.

The body contains long structures called tentacles that can be moved to capture prey.

The tentacles contain stinging cells called cnidocytes and within each one is a capsule called a nematocyst, which discharges to either trap or sting the prey.

Contractile (muscle-like) fibers are found in both the epidermis and the gastrodermis. Their movements are not complex because they do not have a brain

Cnidarians have a hydrostatic skeleton. The contractile fibers act against the fluid-filled gastrovascular cavity. The movements are like a balloon; the animal can be short and thick or long and thin.

VIDEO - Hydra movement, X40, X100 (3.59 MB)

Cnidarians have a saclike gut and extracellular digestion.

Two body forms are found among the Cnidarians, a polyp and a medusa. A polyp is attached and has the tentacles and mouth directed upward. A medusa is free-floating and has the mouth and tentacles on the ventral surface. It resembles an upside-down polyp. Some species have both a polyp and a medusa in their life cycle, others have one or the other form dominant.


Hydra and Relatives (Class Hydrozoa)

Most cnidarians are marine species but a few hydrozoans, including Hydra, live in freshwater.

Members of this group can reproduce by budding, an asexual process in which small polyps form on a parent polyp and then break off.

Below: Budding in Hydra


Sexual reproduction also occurs and produces a larval (juvenile) form called a planula (plural planulae). The planula is ciliated and capable of swimming to a new location. Planulae settle down and develop into a polyps.

Obelia is a colony of polyps enclosed by a protective layer composed of chitin. New polyps are produced asexually by budding. Budding from the polyp also produces Medusae. The medusae function in sexual reproduction by producing sperm and eggs. The zygote develops into a ciliated larvae (planula) which settles down and develops into a polyp colony. The life cycle of obelia is shown below.

Below: Obelia

Below: Obelia (X 40)

Below (2 photographs): Obelia medussae (X100)


Below: hydrozoan jellyfishes (preserved specimens); Gonionemus; Polyorchis 

The Portuguese man-of-war (below) is a colony. The original polyp becomes a float. Other polyps become specialized for feeding or reproduction. The feeding polyps each contain a single long tentacle.

Sea Anemones and Corals (class Anthozoa)

Sea anemones and corals have polyps and no medusae.

Below: Sea anemone (preserved specimen)

Corals are colonial and secrete calcium carbonate skeletons. Coral reefs are the accumulation of these skeletons.

Below: Close-up of the calcium carbonate skeleton secreted by the northern coral (astrangia).

Many corals have photosynthetic algae living within their tissues. Photosynthesis provides these corals with an additional source of food.

Jellyfishes (Class Scyphozoa)

The polyp stage of schyphozoans is small; the medusa stage is dominant. Species that live in the open ocean usually do not have a polyp stage in the life cycle.