Date of Defense


Date of Graduation



Biological Sciences

First Advisor

John Jellies

Second Advisor

Robert Eversole

Third Advisor

Thomas Groves


Visual systems are essential to an organism’s survival. There are a wide range of photosensory organs across the animal kingdom, varying by means of structure, complexity, and the way in which they transduce information. Regardless of variation, visual systems provide the organism with information regarding external stimuli based on how light interacts with surrounding matter. The medicinal leech, Hirudo verbana, is one of the many important model organisms of neurobiology as this segmented annelid possesses a relatively simple nervous system and a rudimentary visual system. The leech also exhibits a restricted range of quantifiable overt behaviors and is capable of adapting to environmental challenges. Consisting of thirty-two segments, the first four segments of the animal are coalesced to form the head, and the last seven segments are combined to form the tail. The photoreceptive organs of the head and body have been established through past research (Kretz et al., 1976), five bilateral pairs of photoreceptive eye cups are present in the cephalic region, and seven bilateral pairs of photoreceptive sensilla are present in each body segment. The photoreceptive organs of the caudal region have yet to be clarified. Behavioral studies have indicated that the animal is responsive, engaging in escape behavior when UV light is shone on the caudal portion (Jellies, 2014). Further, a study done in 2019 isolated the caudal ganglion and its peripheral nerves subjecting the solitary nervous tissue to light stimulation (Spivey 2019). The study concluded that the isolated caudal ganglion exhibit S cell responses to light between 632 nm - 372 nm (Spivey, 2019) in a manner directly comparable to the response seen in body wall sensilla (Jellies, 2014). As it has been established that the tail sucker contains photoreceptive cells, the aim of this study is to establish the presence of photoreceptive organs, such as sensilla, in the tail, and describe their distribution. Hirudo verbena leeches were dissected, and caudal nervous tissue stained with DiI. A confocal microscope was used to view the neuronal structures of the tail sucker. Presumptive photoreceptive sensilla were located. These sensillar organs appeared to be organized in a line of three at the distal ends of eight different nerves branching from the hind brain, for a total of twenty-four caudal sensilla. We have suggested some possible explanations for the caudal sensilla structure and organization.

Access Setting

Honors Thesis-Open Access

Thesis Presentation-1.pdf (18402 kB)
Defense Presentation