The epidermal tissue is a single layer of cells or a layer of fused cells covering a layer of circular muscle above a layer of longitudinal muscle. Flatworms have three embryonic germ layers that give rise to surfaces covering tissues, internal tissues, and the lining of the digestive system. Most flatworms are parasitic, including important parasites of humans. The relationships among flatworms, or phylum Platyhelminthes, is being revised and the description here will follow the traditional groupings. The ecdysozoan phyla have a hard cuticle covering their bodies that must be periodically shed and replaced for them to increase in size. The name comes from the word ecdysis, which refers to the periodic shedding, or molting, of the exoskeleton. The nematodes and the arthropods belong to a clade with a common ancestor, called Ecdysozoa. Finally, the arthropods, one of the most successful taxonomic groups on the planet, are coelomate organisms with a hard exoskeleton and jointed appendages. The nematodes, or roundworms, possess a pseudocoelom and consist of both free-living and parasitic forms. The flatworms are acoelomate organisms that include free-living and parasitic forms.
Associated with bilateralism is the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where the organism first encounters its environment. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are mirror images of each other. The animal phyla of this and subsequent modules are triploblastic and have an embryonic mesoderm sandwiched between the ectoderm and endoderm. Compare the internal systems and the appendage specialization of arthropods.Describe the structural organization of nematodes.Describe the structure and systems of flatworms.Learning Objectives By the end of this section, you will be able to:
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