The sponges and the cnidarians represent the simplest of animals. Invertebrates include a huge diversity of animals, millions of species in about 32 phyla, which we can just begin to touch on here. Although in general we are most familiar with vertebrate animals, the vast majority of animal species, about 95 percent, are invertebrates. The kingdom of animals is informally divided into invertebrate animals, those without a backbone, and vertebrate animals, those with a backbone. Describe the organizational features of cnidarians.Describe the organizational features of the simplest animals.Groups of cells in the nerve net form nerve cords that may be essential for more rapid transmission.Learning Objectives By the end of this section, you will be able to: The function of the nerve cells is to carry signals from sensory cells and to contractile cells. The nervous system is primitive, with nerve cells scattered across the body in a network. However, organs and organ systems are not present in this phylum. There are differentiated cell types in each tissue layer, such as nerve cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as intercellular connections between the cells. Between these two layers is a non-living, jelly-like mesoglea. The outer layer is called the epidermis, whereas the inner layer is called the gastrodermis and lines the digestive cavity. All cnidarians have two tissue layers, with a jelly-like mesoglea between them.Īll cnidarians have two tissue layers. \): Cnidarians have two distinct body plans, the (a) medusa and the (b) polyp.
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