Environment Influences the Body Plans of Organisms

¶ … Environment Influences the Body Plan of Organisms

The distinction between Radiata and Bilateria, or organisms with a radial or bilateral symmetry, is that the latter have a dorsal/ventral polarity resulting in bilateral body axes (reviewed by Martindale and Henry, 1998). By comparison, species with radial symmetry, the Radiata, have a single anterior/posterior axis in their body plan and no dorsal/ventral polarity. Another general distinction is the presence of three germ layers in the Bilateria, which are the endoderm, ectoderm, and mesoderm. These three pluripotent cell types will give rise to organs and internal epithelial layers, skin and nervous tissues, and muscle and connective tissues, respectively.

Role of Environment

Radiata contain a digestive tract that is perpendicular to radial structures (reviewed by Martindale and Henry, 1998). For example, the cnidarians have a mouth that is surrounded by radial pattern of tentacles. After the food enters the mouth, it collects in a gastric cavity shaped like a bowl. The Radiata ctenophores have a complete digestive system resembling that found in vertebrates, with a mouth, esophagus, gut, and anus, but the axis is also perpendicular to the radial structures (tentacles).

By comparison, Bilateria have a nervous system, gut, and in many cases appendages with an anterior/posterior axis of orientation. The evolution of Bilateria is believed to represent a less passive method for finding food, one that depends on a nervous system capable of sensing and reacting to environmental cues through motor commands (reviewed by Yoshimura and Motokawa, 2010). Bilateria are therefore capable of actively moving toward food sources, whereas Radiata are generally forced to wait for the food to come within range of their tentacles. The evolution of bilateral symmetry has allowed exploitation of environmental niches normally out of reach for organisms with a radial symmetry.

Case Study in Bilateral Symmetry Evolution

Sea urchins are members of Radiata; however, sea urchins with bilateral symmetry have emerged at least twice during evolutionary history (reviewed by Yoshimura and Motokawa, 2010). The bilateral sea urchins of the Early Jurassic period had an anus positioned near the end of the body and when engaged in locomotive activity, kept the anus oriented rearward. The slimming of the bilateral sea urchin body shape is believed to have facilitated digging into the sand for food because less sand had to be moved, and the rearward pointing anus more sanitary.