Dark Banquet - Bill Schutt [51]
I nodded, as if I’d been a leech fan all my life. I had to admit (although not to Rudy), that even though I studied vampire bats for a living, leeches had always given me the creeps. In fact they were right up there with clowns and televangelists. Now, sitting in front of this enthusiastic cheerleader for Team Hirudo, I was starting to see his tiny co-workers in a completely different light.
Leeches belong to the phylum Annelida (the segmented worms). The group contains around twelve thousand species and has a worldwide distribution. In addition to leeches (which belong to the class Hirudinea), annelids also include the earthworms (class Oligochaeta) and their freshwater relatives, and as well as marine wigglers like sandworms and bloodworms (class Polychaeta).
Numbering around 650 species,*70 leeches can be found in cool freshwater environments or stagnant tropical mud wallows. About 20 percent of leech species are marine and their habitats range from shallow coastal waters to thermal vents located twenty-five hundred meters below the surface. Other leeches are completely terrestrial and they too exist in a diverse array of habitats: from tropical rain forests to the sub-Himalayan hills of northern India. There’s even an unpigmented species that inhabits a single cave in New Guinea where it feeds on the blood of bats. Although leeches are most famous (or infamous) for their bloodsucking ability, many species are predatory rather than parasitic. A few species even provide some benefit to their rather oblivious hosts.
Annelids range in size from less than a millimeter in length to over three meters long, in the case of a giant Australian earthworm. They’re called segmented worms because their bodies are composed of ring-shaped segments (annuli) stacked one upon another (kind of like the Michelin Man—but with no arms or legs and a bit more slime).
From an evolutionary perspective, the adaptive advantage of segmentation (also referred to as metamerism) appears to be related to the fact that annelid bodies are literally divided into a series of more or less independent sections (walled off from each other by thin septa). In the distant past, these annuli may have served as an early framework for regional specialization of the animal body into a head, midsection, and tail end.
Apparently, segmentation also allowed annelids to move more efficiently than their nonsegmented ancestors. This is because along with a metameric body plan, another adaptation evolved in this group—a body cavity known as a coelom. This fluid-filled chamber is part of the worm’s hydrostatic skeleton, so that when muscles that encircle each body segment contract, the compressed fluid within the coelom is forced toward the head end, projecting the front of the body forward. You can demonstrate this type of locomotion for yourself by squeezing one end of an elongated, water-filled balloon around the middle. Your hand represents the annelid’s circular body muscles, while the water and the expandable balloon interior act as the coelomic fluid and the coelom, respectively. In earthworms, as the body extends forward, the worm secures itself to the substrate with ventrally located pairs of microscopic, toothlike setae (or chaetae). The back end of the body is then pulled forward as longitudinal muscles that run down the length of its body are contracted. The vermiform crawling that results from this anatomical arrangement is quite a bit more efficient than the wild whipping and thrashing movements that characterize other worms like nematodes (commonly known as roundworms).
Leeches, however, are capable of employing an alternative form of locomotion, and in these instances they resemble inchworms (which are insect larvae, not real worms). This type of locomotion is possible because of the presence of a pair of sucker disks—one located near the head end (the anterior sucker) and the other near the tip of the tail (the caudal sucker).*71
During inchworm crawling, the leech attaches its caudal sucker to the substrate (which may be oriented