Explore Animal Diversity with Microscope Slides

By Allan Morrison
Director of Life Science

Animals share certain characteristics.

• They are multicellular, and their cells have nuclei and lack walls.
• They are heterotrophic and generally ingest, rather than absorb, their food.
• Their sexual reproduction produces a zygote that develops into a blastula (a ball of cells) before developing further. (A blastula stage during development is a defining feature of animals.)
• All but the very simplest animals have nervous and muscle tissue.

Within those parameters, diverse animal groups reveal an array of body plans and structures and levels of complexity. With prepared microscope slides, your students can see the general and the particular characteristics of animals. (For example, they see multicellularity but distinct differences in organizational complexity of the cells.) Using whole mounts or prepared sections of small animals, students can observe a sponge (an aggregate of loosely organized, specialized cells), a Hydra (with distinct tissues), and a planarian (with distinguishable organs and the suggestion of a "head"). Using the same specimens, students see asymmetry, radial symmetry, and bilateral symmetry.

During discussions of animal diversity and evolution and classification, students might view the trend toward cephalization, from the planarian's simple eyespots to the complex sensory structures at the anterior of a Daphnia, a mosquito, or a fruit fly. Students can easily view structural adaptations within animal groups (Hydra and Obelia, mosquito and fruit fly, or Daphnia and Cyclops). These comparisons demonstrate the diversity that has developed within the specific parameters that define a taxon or a clade.

With some specimens, students can create live mounts to use in conjunction with prepared slides. Then, the students see structures in action and better understand function (e.g., tentacles of Hydra, various organs in Daphnia, the locomotion of annelid worms).

The following photos from some of Carolina's prepared slides or live mounts of our small animal specimens illustrate how microscope slides can enhance instruction about animal diversity.

Grantia

Sponges belong to the phylum Porifera. Sponges have no true tissues; the body is made up of cells loosely organized by type. Sponge cells occur in a few basic types. Flat epidermal cells called pinacocytes, surround pores in the body wall. Choanocytes, or collar cells, create a current with the synchronized beating of their individual flagella. This current draws water through pores into the central cavity of the sponge, called the spongocoel. Water constantly channels through the sponge, carrying suspended food particles. Amoeboid cells in the noncellular matrix of the body wall engulf and digest food and distribute nutrients to the other cells. Water exits the sponge through the osculum.

Hydra and Obelia

Hydra and Obelia are in the phylum Cnidaria. Two basic body plans seen in this phylum are a swimming medusa (e.g., a typical jellyfish) and a sessile polyp. (Although polyps generally do not move, Hydra is capable of moving slowly along a substrate by a succession of somersaults.) Obelia occurs in 2 different forms: colonial polyps and reproductive medusae.

Cnidarians have 2 true tissue layers, an outer epidermis and an inner gastrodermis. A noncellular matrix called the mesoglea makes up the bulk of the animal. This material is found between the 2 tissue layers. It serves as a storage site for nutrients.

The gastrodermis lines the gastrovascular cavity, the central cavity of the polyp where food is digested. Because undigested remnants exit through the mouth rather than a separate opening, the type of digestive system in cnidarians is termed "incomplete."

The mouth is surrounded by tentacles used for gathering food. Stinging cells called cnidocytes, which can incapacitate prey, appear as bumps on the tentacles.

Figure 2  Living Hydra, capturing Daphnia.



Figure 3  Hydra, whole mount (item #306040).



Figure 5  Obelia polyps, whole mount (item #306124).



Figure 6  Obelia medusa, whole mount.

Planarian

Planarians are aquatic flatworms belonging to the phylum Platyhelminthes. Planarians exhibit bilateral symmetry and a degree of cephalization, i.e., the development of nervous tissue and sensory structures at the anterior of the body. Lateral projections called auricles are sensitive to touch and to chemicals dissolved in the water. The 2 eyespots are sensitive to light but cannot form images.

Like Hydra, the planarian has an incomplete digestive system, a gastrovascular cavity with only 1 opening. The pharynx is located mid-ventrally. This muscular tube extends from the body and is used to suck up food particles. The pharynx empties into the gastrovascular cavity.

This cavity is elaborately branched and occupies most of the planarian's body. Food is digested within the cavity, and waste exits the pharynx. Planarians have no secondary body cavity, or coelom, and are therefore called acoelomate animals. Planarians move by means of cilia located on the ventral surface. Mucus secretions aid movement.

Figure 7  Planarian, whole mount (item #306312).

Figure 8  Planaria Combination, whole mount (item #306324).

Extension

1. Have students use microscope slides in a study of the life cycles of animals in different taxonomic groups. Students might examine slides of different reproductive forms and structures.
2. Have students examine several microscope slides to better visualize acoelomate, pseudocoelomate, and coelomate body plans. Whole mounts and sections of a variety of animals show the tissue layers and cavities very clearly.