Fortunately, the under either classification the term hirudinea covers virtually all the leech diversity, because all but a few species are found in the same subclass. People generally have a strong negative reaction to the mention of leeches, and identify them only as blood suckers. It is also not generally known that there are many terrestrial leeches. I am told, cluster in such numbers on the legs of animals that travel on foot is nearly impossible. Some tropical species climb tall vegetation and await movement below them to drop from their perch and attach to the passer-by.
Leeches have a fixed number of segments in their bodies.
The number of segments is remarkably constant for a group with a great deal of variation in size. Although leeches are found in a wide variety of habitats, they are found primarily in shallow, vegetated sluggish streams or standing lakes and ponds. The group in which most people are interested, I am sure, are the polychaetes. On the other hand, many sedentary species construct and live in semi-permanent burrows or tubes of various complexity, but some species can leave the burrow or tube and start a new home. Minneapolis, Minn. 263 pp.