function of myelin sheaths; unlike demyelination, it does not produce lesions. such defective sheaths often arise from genetic mutations affecting the biosynthesis and formation of myelin. the shiverer mouse represents one animal model of dysmyelination. human diseases where dysmyelination has been implicated include leukodystrophies (pelizaeus–merzbacher disease, canavan disease, phenylketonuria) and schizophrenia. invertebrate myelin functionally equivalent myelin-like sheaths are found in several invertebrate taxa including oligochaetes, penaeids, palaemonids, and calanoids. these myelin-like sheaths share several structural features with the sheaths found in vertebrates including multiplicity of membranes, condensation of membrane, and nodes. however, the nodes in vertebrates are annular; i.e. they encircle the axon. in contrast, nodes found in the sheaths of invertebrates are either annular or fenestrated; i.e. they are restricted to "spots". it is notable that the fastest recorded conduction speed (across both vertebrates and invertebrates) is found in the ensheathed axons of the kuruma shrimp, an invertebrate, ranging between 90 and 200 m/s (cf. 100–120 m/s for the fastest myelinated vertebrate axon).