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Medicinal leeches gather visual input from both its eyecups and photoreceptors across its body surface; each of its 21 midbody segments possesses 14 photoreceptive sensilla. It has been previously established that the posterior sucker is composed of seven body segments that are fused during embryogenesis. Similarly, four body segments fuse to create the anterior brain. There are five bilateral pairs of eyes located on the dorsal surface of the leech’s head and seven bilateral pairs of sensilla distributed over the surface of each midbody segment (Kretz et al., 1975). If no sensilla were lost during the fusion of the seven body segments that make up the posterior sucker, it would possess in the order of one-quarter of the receptors in the entire body. It has been observed that leeches have different responses to ultraviolet light stimulation on the anterior and posterior portion of their bodies suggesting that leeches are responsive to stimuli at both their head and tail (Jellies, 2013). The presence of photoreceptors on the tail sucker could contribute to the difference in the observed behaviors. There have been few studies on the hindbrain and tail sucker of the Medicinal Leech, however, there are known pathways in which the hindbrain and anterior brain communicate that play a role in rhythmic and mesenteric movements like crawling and swimming (Baader et al., 1997). In 10 trials, this study isolated the caudal region of the leech through partial ablation of the connective nerve between ganglion eleven and twelve and division of the body wall between body segment twelve and thirteen, then tested visual responses with and without the hindbrain isolated. Extracellular recordings were analyzed for S-cell response then standardized and compared. All 10 trials showed S-cell response to light stimuli while the hindbrain was isolated suggesting the tail sucker possesses sensilla.
Spivey, Claire, "The Presence Of Photoreceptor Sensilla In The Posterior Sucker Of Medicinal Leeches Inferred Through Functional Visual Response." (2019). Honors Theses. 3133.
Honors Thesis-Open Access