The coelom differs from that of other annelids in that it is largely filled in with tissue. The blood of some leeches is red. Gas exchange occurs through the body surface of most leeches, although many fish-parasitizing leeches have gills. The sense organs consist of sensory cells of various types, including photoreceptor cells, scattered over the body surface. There are also from 2 to 10 eyes, consisting of clusters of photoreceptor cells and located toward the front of the body.
Many parasitic leeches are able to parasitize a wide variety of hosts.
Most of the marine and some of the freshwater leeches are fish parasites. The medicinal leech, Hirudo medicinalis, is one of a group of aquatic bloodsucking leeches with jaws. The digestive tract of bloodsuckers produces an anticoagulant, hirudin, which keeps the engorged blood from clotting. A few leeches attach permanently to the host, leaving only to reproduce. Predatory leeches are active at night and hide by day.