They are bilaterally or radially symmetrical. These clefts can occur bilaterally or unilaterally. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. The nerve roots then merge into bilaterally symmetrical pairs of spinal nerves. They have a pseudometamerism of bilaterally symmetrical repeated organs and muscles. The condition is commonly unilateral but can also occur bilaterally. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons. Brown bullheads are ectothermic, heterothermic, and bilaterally symmetrical. Early echinoderms were bilaterally symmetrical, as their larvae still are. There are two centromedian nuclei arranged bilaterally. Echinoderm larvae are not included, since they are bilaterally symmetrical. The Annelids are bilaterally symmetrical, triploblastic, coelomate, invertebrate organisms. Potential predators were bilaterally symmetrical, as were potential prey. This type of dislocation may happen bilaterally or unilaterally after yawning.