Activity 1: Background Information

The Biosphere
Spread out over the Earth’s surface, between the solid rocky crust of the inner Earth, and the upper reaches of the atmosphere, extends the world of life–a film of living matter. It is made up of grass, shrubs, and trees, of micro-organisms, of worms, fish, rabbits, and wolves. This thin layer of life, along with the air, soil, water, and other nonliving matter that surrounds it, is called the biosphere.

Organisms exist that live in almost every conceivable environment. There are algae that grow only on melting snow. Many plants and animals spend their entire lives in lightless caves. Some algae and bacteria live in hot springs where temperatures are near the boiling point. That all these habitats are populated is evidence of genetic variability and natural selection.

That is the focus of this chapter of Active Biology, and the first activity introduces students to the incredible biodiversity of the animal kingdom. This activity is not an attempt to walk the students through the animal phyla. Its purpose is to arouse the students’ interest in the diversity of life, develop their observational skills, and perhaps to encourage them to generate questions that they may wish to pursue in other biology courses.

Biodiversity in the Animal Kingdom The animal kingdom presents an enormous diversity of organisms. Although they display many structural and functional differences, all animals share certain characteristics. All are essentially multicellular in organization, and heterotrophic. Most are capable of locomotion for some part of their lives, and most are able to respond rather quickly to changes in their environment.

Scientists divide this enormously complex and large kingdom into two large groups: the invertebrates and the vertebrates. The invertebrates do not have a backbone, and the vertebrates have a “backbone” (notochord) for at least some time in their lives. Students will be asked to consider if the organisms they are observing are invertebrates or vertebrates. Students may also find it interesting to realize that over 95% of the described animal species are invertebrates!

Complexity and evolutionary development can also be looked at from the point of view of the body cavity. Although the body cavity, called the coelom, is important in classifying animals, it is not within the scope of this biology course.

Zygote (the cell resulting from the union of male and female sex cells) development provides another way of dividing animals into phyla, but is also not examined in this activity.

Symmetry also provides clues to the complexity and evolutionary development of an organism. More complex animals display bilateral symmetry. Students will have an opportunity to view organisms that display both radial and bilateral symmetry.

The simplest organism that the students will encounter in this activity is the hydra, from the phylum Cnidaria. (Polifera, or the sponges, are even less complex animals, and at one time were considered to be plants.) The Cnidaria include hydra, jellyfish, and coral. Members of this phylum, which consists of about 10,000 species, are found only in aquatic ecosystems. Cnidarians have only two germ layers, the ectoderm and the endoderm. However, unlike sponges, they have true tissues: nerve, muscle, and digestive.

The planarian, a member of the phylum Platyhelminthes, or the flatworms, represent a further increase in complexity. Planaria show bilateral symmetry and have a primitive brain. They also have true organs and rudimentary organ systems for digestion
and excretion.

The phylum Annelida, or the segmented worms, possess a true coelom. Segmentation is also an important evolutionary advantage that annelids share with humans. Segmentation permits greater specialization. The earthworm is an annelid from the class Oligochaeta. Other annelids include leeches and bristled marine worms.

Arthropods are the pinnacle of adaptation and diversity. They are a large group of animals that can be found in all habitats. They include: spiders, scorpions, ticks, mites, lobsters, crabs, crayfish, centipedes, millipedes, and 750,000 species of insects like beetles, flies, ants, bees, wasps, moths, and butterflies. The key feature separating arthropods from other species is the exoskeleton. More complex arthropods show an increase in brain size and complexity, and thus an increase in sensory abilities. Students will note that hermit crabs have eyes and antennae with which they sense their environment.

Unlike invertebrates, the chordates are organisms that at some point in their development have a stiff rod, the notochord, running down their back. About 95% of chordates belong to the subphylum Vertebrata. Vertebrates have a hollow, bony structure that surrounds the dorsal nerve cord. They also have an endoskeleton, a large brain protected by a skull, a complex heart and circulatory system, an advanced nervous system, and a large coelom that contains the vital organs. Frogs are amphibians. They are born in fresh water and live initially as gilled tadpoles. They then change into the adult form that the students will be observing.
The adult is air-breathing and lives on land.