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Genes, nerves, and cell types link cnidarian sea anemones to invertebrate annelid worms, arthropods (insects), and mollusks. Sea anemones have a nerve net, muscles, and body plan genes, which were the origin of the nervous and muscle systems of moving animals.
Cnidarians include coral, jellyfish, and sea anemones. They all have two germ layers, the endoderm and the ectoderm (epidermis). The digestive cavity (stomach in Figure 8-20) connects to all parts of the body, so they do not need a blood system or other mechanism to deliver nutrients to cells. Cnidarians do not have brain or a nerve cord, but their nerve net allows control and detection signals from cells on one side of the body to reach cells on the other side of the body. Some jellyfish have sense organs that detect odor, light, and vibrations. Jellyfish begin their life cycle as polyps anchored to the ground, but they detach from the ground and become jellyfish where their muscles allow them to move through the water by expanding and contracting. They are the most efficient swimmers in the world.
Figure 8-20. Cross-section of a jellyfish. Credit: Marianna Ruiz Villareal. Public domain.
The cavity elongated and the body became bilateral in flatworms (planaria), which were possibly the first bilaterian animals. They are acoelomates (video picture below), which means that they have a cavity for digestion, but no cavity outside the gastrovascular cavity. They have nerve cords running down each side of the body and transverse nerves running across the body. They also have a cluster of neurons in the head and light sensitive spots. They have three germ layers, which means that they can develop more complex tissues than cnidarians. As with cnidarians, they have a gastrovascular cavity that delivers nutrients to all cells in the body and have no circulatory or respiratory system. They also do not have an anus so waste must be expelled through the mouth, as with cnidarians.
Figure 8‑21. Anatomy of a nematode. Credit: K.D. Schroeder. Used here per CC BY-SA 3.0.
The next probable step in animal evolution were the pseudocoelomates (Figure 8-21), which includes rotifers and nematodes (roundworms). They have a complete digestive system with an anus. They are narrow and nutrients can pass through the digestive tract to all cells in the body so there is no need for a circulatory or respiratory system.
Figure 8-22. Reconstruction of the Burgess Shale fossil polychaete Burgessochaeta setigera. Credit: Obsidian soul. Used here per CC BY 3.0.
The coelemates followed the pseudocoelomates. They have complex organs, circulatory systems, and nerve systems. The two branches of coelomates are the protostomes and deuterostomes. One branch of protostomes are the annelid worms, which include marine ragworms (polychaete) and earthworms (oligochaete). The polychaete, marine ragworms (Figure 8‑22), first appeared in the fossil record in the middle Cambrian Period, in the Sirius Passet Lagerstatte in Greenland, 518 Ma, at approximately the same time as the other crown coelomates (i.e. trilobites).
Annelid worms have concentrations of neurons at the anterior (front) and posterior (rear) ends, and a double ventral nerve cord that runs along the length of the body. In addition to a mouth, esophagus, and anus, they have several organs in the digestive system, including a crop (storage), gizzard (grinding), and intestine. The earliest annelids might have been polychaete worms (marine ragworms). They have bristles on the side (Figure 8‑22), which they use for swimming. Some annelids have gills in each segment for gas (oxygen and carbon dioxide) exchange with the environment while others rely on gas exchange through the skin. This is possible because they are generally long and narrow and thus no cell is extremely far from the skin. Annelids have a well developed circulatory system. Most species do not have a heart because the blood is pushed by contracting blood vessels; however, a few species have muscle pumps that pump the blood through the animal, similar to a heart. The reason that annelid worms need an efficient circulatory and respiratory system is that they are highly mobile and require efficient nutrient and oxygen transfer to muscles. Annelid worms are segmented, and septa divide sections in annelid worm into distinct segments. The blood vessels and nerve cord pass through the septa, but the muscles do not. Annelids continued to evolve on land. Earthworms (Figure 8‑23) are oligochaete annelid worms.
Figure 8‑23. Cross-section of oligochaete annelid worm body plan. Credit: KDS44. Used here per CC BY-SA 3.0.
During the last few decades, the Arendt lab discovered many genetic similarities between vertebrates and annelid worms. Arendt discovered that 2/3 of the introns found in vertebrate DNA are found in the marine ragworm, Platynereis; however, most are not found in insects (arthropods).[1] Many leading researchers to not think that vertebrates and annelids are closely linked because of the many differences between annelid worms and vertebrates.[2] One of the arguments against a close ancestral connection between vertebrates and annelid worms is that vertebrates and other deuterostomes have a radial embryo cleavage pattern in the early phase of embryogenesis while annelids are protostomes, which have the opposite orientation in the embryo stage and have spiral cleavage patterns. However, Arendt has shown that the differences in orientation are not as great as previously thought because there is a somewhat sideways orientation in early development.
References
References
[1] Raible F., K. Tessmar-Raible, K. Osoegawa, P. Wincker, C. Jubin, G. Balavoine, D. Ferrier, V. Benes, P. de Jong, J. Weissenbach, P. Bork, D. Arendt. 2005. Vertebrate-type intron-rich genes in the marine annelid Platynereis dumerilii. Science 310(5752): 1325–26.
[2] Gee, Henry. Across the bridge: understanding the origin of the vertebrates. University of Chicago Press, 2018.
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Sea anemones. Credit: Giancarlo Merculiano (1893)
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