The star of the lab is the medicinal leech, Hirudo medicinalis. This is an annelid worm with specialized suckers at head and tail. The body of the worm is segmented and this segmentation is very clear internally. Each leech is composed of 32 segments. Internally the CNS is composed of clusters of neurons (ganglia), one per segment. The anterior 4 ganglia are fused into a large subesophageal ganglion, the posterior seven are fused into a large tail ganglion. Each ganglion contains about 200 different neurons and most are bilaterally paired and individually identifiable. One of the many identified neurons we study are the Heart Excitor neurons (above right and left).
|Shown here is a hemisegment of a leech stained to reveal axons. The ventral midline is at the left, dorsal at the right, anterior is upward. Notice the regular arrangement of nerves. This same pattern is repeated segmentally. The nerves also contain the axons of many peripheral neurons that project toward the CNS and between segments.|
|H. medicinalis is an obligate blood feeder. Shown here are a number of worms at the dinner table. In our case, we obtain bovine blood from a commercial supplier and put it into sausage casings for the leeches to feed on.|
|The worms are hermaphrodites that live many years. To breed them in captivity we control feeding, light cycles, temperature and environment. Leeches, like other annelids deposit fertilized eggs into cocoons which are gathered and tracked individually.|
|The leeches would normally develop for about 30 days inside of the cocoon. Because they each carry their own albumin supply they can be isolated from the cocoon early in development. We rear many of these animals and return them into the colony.|