A Few Select Topics
General Hirudo Development
Like all of the annelid worms in Class Clitellata these leeches lay cocoons. Gravid leeches leave the water to deposit their cocoons on land. The parent leech has deposited into each cocoon about 20 synchronously developing sibling cryptolarvae along with an albuminous yolk that the larval form of the leech eats. This provides the nutrients and energy for the embryonic form to grow.
YouTube movie clip of live leech embryos.....click the cocoon picture at the right.
Development within the cocoon proceeds for 30 days and leeches in this Order undergo a type of metamorphosis within the cocoon. There is a larval stage that is never free-living and co-exists with the developing embryonic form. This is a musclar larval sac that has a nerve net, it's own mouth and pumping pharynx as well as excretory organs. The embryo develops as a superfical sheet on top of the larva, deriving nutrition and structural support from it. The embryonic sheet is termed a germinal plate and is transparent for about 2/3 of development, allowing for all of the developing cells to be visualized easily.
The germinal plates mature over several weeks, expanding both longitudinally and laterally. Notice that eventually the edges of the plate wrap around the larval form and engulf it. The dorsal margins zipper together to form a tubular worm, entirely engulfing the larval form in the developing gut of the leech. At about 30 days, the juvenile worms leave the cocoons and crawl toward the water where they will swim away.
