Introduction to Animals
Animals are multicellular
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.
Animals have a period of embryonic developmentDuring 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
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
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.
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.
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.
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 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 AnimalsSmall 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.
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.
Some Anatomical Terms
Sponges (Phyla: Calcarea and Silicea)
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.
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.
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.
Some examples of Cnidarians are hydra, jellyfishes, corals, sea anemones, and Portuguese man-of-wars.
The photograph below is a hydra (40X).
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 (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).
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.