If present in great numbers on a host, these can be debilitating, and in extreme cases, cause death.Leeches are unusual in that they do not produce certain digestive enzymes such as amylases, lipases or endopeptidases. A deficiency of these enzymes and of B complex vitamins is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hirudo medicinalis, these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, Aeromonas veronii. Non-bloodsucking leeches, such as Erpobdella octoculata, are host to morew bacterial symbionts. In addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut. This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow. Nervous system
A leechs nervous system is formed of a few large nerve cells; their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems. The main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. A nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments six to 26. In segments 27 to 33, other paired ganglia fuse to form the caudal ganglion.
In the case of the North American Erpobdella punctata, the clutch size is about five eggs, and some ten cocoons are produced. Each cocoon is fixed to a submerged object, or in the case of terrestrial leeches, deposited under a stone or buried in damp soil. The cocoon of Hemibdella soleae is attached to a suitable fish host. The glossiphoniids brood their eggs, either by attaching the cocoon to the substrate and covering it with their ventral surface, or by securing the cocoon to their ventral surface, and even carrying the newly hatched young to their first meal.When breeding, most marine leeches leave their hosts and become free-living in estuaries. Here they produce their cocoons, after which the adults of most species die. When the eggs hatch, the juveniles seek out potential hosts when these approach the shore. Leeches mostly have an annual or biannual life cycle. Feeding and digestion
About three quarters of leech species are parasites that feed on the blood of a host, while the remainder are predators. Leeches either have a pharynx that they can protrude, commonly called a proboscis, or a pharynx that they cannot protrude, which in some groups is armed with jaws.In the proboscisless leeches, the jaws (if any) of Arhynchobdellids are at the front of the mouth, and have three blades set at an angle to each other. In feeding, these slice their way through the skin of the host, leaving a Y-shaped incision. Behind the blades is the mouth, located ventrally at the anterior end of the body.