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Opisthokonta: Metazoa II: Eumetazoa: Bilateria: Protostomia: Ecdysozoa Objectives: • Know the key features of the Phylum Nematoda. Know how they relate to other metazoan phyla. • List the key characteristics of the Phylum Arthropoda. Identify examples. List the four subphyla and their key distinguishing features. Identify examples of the subphyla Chelicerata, Crustacea, Myriapoda, and Hexapoda. • Be able to identify the different structures on these organisms and understand their function. • Understand differences between terrestrial versus aquatic arthropods (e.g., grasshoppers versus crawfish) and how that relates to environment in which they live. • Define and be able to apply the terms in bold found throughout this exercise. Introduction: Our survey of metazoan diversity continues with the clade Bilateria, which consists of animals which have bilaterally symmetrical body plans, unlike the radially symmetrical Phylum Cnidaria and the largely asymmetrical Phylum Porifera. Additionally, members of this clade are triploblastic, meaning that they have three embryonic tissue layers from which structures and organs develop, in contrast to the diploblastic cnidarians. The clade Bilateria can itself be divided into two additional clades based on differences in developmental pathways: protostomes and deuterostomes. At an early stage of embryonic development, around the 128–cell stage, eumetazoans form a hollow sphere of cells referred to as a blastula. In the next developmental step, cell divisions on one side of the sphere result in inward growth toward the hollow interior of the sphere. This developmental step is known as gastrulation, the embryonic stage is known as the gastrula, and the initial opening on the outside of the sphere of cells where the inward growth is known as the blastopore. In most bilaterian animals, the inward growth continues and emerges as a second opening on the opposite side of the gastrula. In protostomes, the blastopore will develop into the mouth of the juvenile and adult stages of the organism. The name is derived from the word roots which mean ―first mouth‖, i.e. the mouth is formed first. The second opening formed becomes the anus. The situation is reversed in deuterostomes (―second mouth‖). Evidence supports division of the clade Protostomia into two clades: Ecdysozoa and Lophotrochozoa. The clade Ecdysozoa comprises one of the most species-rich groups within the animal kingdom and includes eight phyla. This includes the Phylum Arthropoda (including

insects, arachnids, and crustaceans) and the Phylum Nematoda (commonly called nematodes or roundworms). Many of the animals you previously examined, such as sponges and corals, and many that you will examine in the future, such as chordates, build their bodies’ skeletal support from minerals like calcium carbonate or silica. A unique feature of ecdysozoans is that they instead build a cuticle, a non-living, outer layer of organic material that functions as its skeleton. The cuticle serves to both support and protect the animal. It can be built thinner and lighter than in other animals, and does not require a source of minerals for its construction. The fact that a cuticle can be very thin means it does not require joints like a mineral skeleton in order to allow flexibility. The name Ecdysozoa refers to the fact that members of this group regularly shed their cuticle, a process called ecdysis that is controlled hormonally by a class of steroids appropriately called ecdysteroids. Although their name is based upon a morphological trait, there is much molecular data to support the grouping of ecdysozoans into a monophyletic clade, such as the presence of similar ecdysteroids in nematodes and arthropods. If you have ever seen an insect crawl out of its old skin, or a butterfly leaving its chrysalis, then you have witnessed ecdysis. This ability to shed the outer skeleton has opened up developmental options for ecdysozoans that other animals with...
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