Studying Animal Body Plans
Within the kingdom Animalia, there is great diversity. As with other taxonomic kingdoms, animals can be subdivided on the basis of shared characteristics thought to indicate evolutionary divergence and degrees of relatedness. This kingdom includes an array of body plans, structures, and levels of complexity. With the Animal Body Plan Microscope Slide Set, students explore different characteristics of body plans through viewing 6 organisms.
Background information
Animals share certain characteristics: they are all eukaryotic and multicellular; they lack cell walls; they are heterotrophic; and they develop from blastocysts. All but the simplest animals have nervous and muscle tissue, which are found in no other kingdom of life.
Varying levels of complexity exist within the animal kingdom.
Sponges, such as Grantia, are considered the least complex animals, showing relatively little cellular organization. Sponges are made up of aggregates of specialized cells, rather than the more organized tissues found in more complex animals. Hydra reveals a true tissue level of organization. The basic types of animal tissues develop from germ layers, layers of specialized cells produced during the gastrula stage of development. (Sponges have no true germ layers.) Hydras and most other cnidarians derive from 2 germ layers, the ectoderm (outer layer) and endoderm (inner layer), and are referred to as diploblastic. Diploblastic organisms tend to have an outer membrane and an inner membrane, with acellular material in between (e.g., jelly-like mesoglea). Animals that derive from 3 germ layers are termed triploblastic.
Triploblastic organisms may be grouped according to their coelom, the body cavity between the digestive tube and the outer body wall. The acoelomate animals have no such cavity. For example, a planarian’s body is packed with muscle tissue. Coelomate animals’ body cavity is formed in the mesoderm during development and is lined with a specialized layer called peritoneum. Many internal organs may be suspended in the coelom. Earthworms, starfish, and vertebrates are coelomates. In some animal groups, a body cavity forms during development but does not possess its own specialized lining. Such a cavity is called a pseudocoel. Ascaris is a pseudocoelomate animal.
Another important defining feature among animal groups is the digestive pathway. In some simple animal groups, the digestive system is called incomplete and consists of a cavity with a single opening through which food enters and waste exits. Hydras and planarians have incomplete digestive systems. More complex animals have a complete digestive system—a tube running from mouth to anus—which allows for a one-way flow of food. Ascaris has a simple complete digestive system. In the coelomate groups, digestive tubes are not only complete but are more compartmented, with specialized divisions such as the pharynx, stomach, and intestine, and often with several accessory organs such as digestive glands.
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