This image shows the nervous systems of a medical leech and a human
Sensing the Environment

As mentioned in the Habitat section, H. medicinalis mostly lives in freshwaters, and less often on land. What adaptations has the leech developed to survive in these conditions? How does the leech sense its environment? Read on to find out.

Cephalization, segmentation, and bilateral symmetry are major accomplishments in the evolutionary advancement of animals and have allowed annelids with several advantages. Bilateral symmetry and segmentation have allowed certain parts of the body to become specialized, allowing for organs and areas of the body to become more apt to the environment. Segmentation has led to cephalization—the concentration of nervous tissue at one end of the body. Cephalization is extremely important since it allowed for the development of ganglia and a nervous system in the annelids.  

 As the picture to the left shows, the nervous systems of a leech and human have some similarities. The most obvious is the central nervous tissue and ladder-like nerve network. Hirudo medicinalis has a central nervous system consisting of 34 pairs of ganglia that branch off from it. The motor nerve cells are located in the ventral nerve cord and the sensory nerve cells are concentrated in or around the sense organs.

The leech senses its environment in several ways. H. medicinalis has five pairs of "eyes" around its head, but are pretty simple in structure and function compared to ours. They have no lens, rods or cones, but only light-sensitive cells that allow them to differentiate light and dark. The leech also has chemoreceptors near their head that allow them to sense different chemicals. They have large concentrations of nerves in their epidermis that allow them to detect movement in water as well as touch.

All of these adaptations are important in the leech’s survival. First, the light receptors allow them to know their position in the water. If they sense more light, it probably means they are close to the surface of the water, a possibly dangerous place since a bird or another animal could easily come and grab it. If a cow or some other animal came near a pond where the leech was waiting, it could see its shadow and approach it for a meal. So, although the eye of the leech is simple, it serves an important function and aids in its survival. Second, the chemoreceptors in the leech allow them to detect possible food sources. These chemoreceptors may be analogous to our nose and the way we smell different chemical compounds. The leech can "smell" different prey and can close in on them for lunch. Also, their epidermis is lined with sensitive nerves that allow them to detect vibrations from possible prey in the water. If they sense some type of disruption in the water, they often go to investigate it and use their chemoreceptors to determine whether or not it is edible. 

Locomotion: Round, Round Get around, I get a round, YEAH!

I'm an avid fisherman, and I often use leeches from the local bait shop to attempt to catch the "big ones". Leeches have  adapted a unique wave-like motion that they use to swim through the water and fish like northern pike and large mouth bass are often attracted by it. H. medicinalis also uses this characteristic motion to get around in the water quite elegantly. Below is a YouTube video that shows this movement.

This drawing illustrates the leech's looping movement. Courtesy of BIODIDAC

Leeches also utilize a type of "inching" movement that allows them to move along the bottom of a pond or lake, and also on land. They relax their muscles, making their body stretched out and elongated. The leech can then grab on to the substrate in front of them with their sucker, and then contract their muscles bringing forth their backside. Although this movement isn't nearly as fast as their wave-like swimming, it serves the leech with a good purpose, allowing them to be motile in water and on land. The picture to the right illustrates this type of movement. 

                                                        This drawing illustrates the leech's looping movement. Photo courtesy of BIODIDAC

Escaping Predators

As mentioned above, leeches like Hirudo medicinalis are sometimes on the top of the menu for many different types of organisms. Of course, no organism wants to be dinner and most have developed ways to avoid getting eaten. Some organisms have poison, spikes, claws, and scary mannerisms that help them out. However, in the case of leeches, their main line of defense is quite simple--they hide. They spend most of their time buried in the muddy bottom of a pond to avoid attracting their enemies, and in most cases it works for them.

                                       Click HERE to learn about what Hirudo medicinalis eats

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