The coelom is a cavity that separates the ectoderm from the endoderm. This developmental pattern is called protostomy or “first mouth.” Protostomes include acoelomate, pseudocoelomate, and eucoelomate phyla. Lophotrochozoa are also protostomes, in which the blastopore, or the point of invagination of the ectoderm (outer germ layer), becomes the mouth opening into the alimentary canal. In these phyla, we also see the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism-exactly where a mobile bilaterally symmetrical 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 superficially symmetrical. Explain the key features of Platyhelminthes and their importance as parasitesĪnimals belonging to superphylum Lophotrochozoa are triploblastic (have three germ layers) and unlike the cnidarians, they possess an embryonic mesoderm sandwiched between the ectoderm and endoderm.Identify an important extracoelomic cavity found in Nemertea.

Describe the unique anatomical and morphological features of flatworms, rotifers, and Nemertea.By the end of this section, you will be able to do the following:

Flat worm phylum