Annelids, like many other invertebrate animals, replace lost body parts in a process called regeneration. However, the ability to regenerate lost segments is present in some groups and not others; for example, leeches do not regenerate lost segments. Anterior and posterior regeneration involves the formation of a bud containing stem cells that differentiate into the new head or tail segments. Annelid regeneration also involves remodelling of surviving body fragments. The ability of annelids to regenerate tail segments appears to be nearly universal among species capable of regeneration. The ability to regenerate head segments, although common, is not universal and can depend on the number of segments lost. The absence and presence of regeneration across annelid groups, including closely related species, suggests that regeneration ability may be an ancient trait that has been lost in some species during annelid evolution. Key Concepts Annelids vary in their capability for regenerating body segments, including among closely related species. The ability of annelids to regenerate posterior segments appears to be nearly universal. The ability of annelids to regenerate anterior segments, although common, is not universal and is often limited depending on the number of segments lost. Annelid regeneration may involve both epimorphic and morphallactic mechanisms. Multiple losses and gains of regeneration ability have likely occurred during annelid evolution. Why regenerative ability among annelids varies extensively remains unclear. With development of new techniques for genetic analysis and microscopy, annelids are becoming important model systems for the study of regeneration.
The interneurons associated with rapid escape circuits are adapted for fast pathway activation and rapid conduction. An essential aspect of fast activation is the processing of sensory information with limited delays. Although aquatic annelid worms have some of the fastest escape responses in nature, the sensory networks that mediate their escape behavior are not well defined. Here, we demonstrate that the escape circuit of the mud worm, Lumbriculus variegatus, is a segmentally arranged network of sensory interneurons electrically coupled to the central medial giant fiber (MGF), the command-like interneuron for head withdrawal. Electrical stimulation of the body wall evoked fast, short-duration spikelets in the MGF, which we suggest are the product of intermediate giant fiber activation coupled to MGF collateral dendrites. Since these contact sites have immunoreactivity with a glutamate receptor antibody, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dion abolishes evoked MGF responses, we conclude that the afferent pathway for MGFmediated escape is glutamatergic. This electrically coupled sensory network may facilitate rapid escape activation by enhancing the amplitude of giant axon depolarization.
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