There are many genera and species of leeches, the exact definitions of which are still in need of a more complete survey.
In one subdivision of the leeches, the Gnathobdellidae, the mouth has three chitinous jaws which produce a triangular bite, though the action has been described as like that of a circular saw.
As unlicensed blood-letters, certain land-leeches are among the most unpleasant of parasites that can be encountered in a tropical jungle.
Small leeches taken into the mouth with drinking-water may give rise to serious symptoms by attaching themselves to the fauces and neighbouring parts and thence sucking blood.
All leeches are very extensile and can contract the body to a plump, pear-shaped form, or extend it to a long and worm-like shape.
They are divisible into the Haplodrili or Archiannelida, the Polychaeta containing the marine worms, the Oligochaeta or terrestrial and fresh-water annelids (see Earthworm), the Hirudinea or leeches (see Leech), and a small group of parasitic worms, the 11-Tyzostomida (q.v.).
These chitinous, rod-like, rarely squat and then hook-like structures are found in the majority of the Chaetopoda, being absent only in certain Archiannelida, most leeches, and a very few Oligochaeta.
The division and, indeed, partial suppression of the coelom culminates in the leeches, which in this, as in some other respects, are the most modified of Annelids.
In the leeches, however, there seems to be the commencement of the formation of a syncerebrum.
The capillaries sometimes (in many leeches and Oligochaeta) extend into the epidermis itself.
It has been held that the condition shown in certain leeches tend to prove that the coelom and haemocoel are primitively one series of spaces which have been gradually differentiated.
These characters are plain in all the cases cited, excepting only the leeches which will be considered separately.
A consideration of the mode of development and appearance of the coelomoducts that have thus far been enumerated (with the possible exception of those of the leeches) seems to show that there is a distinct though varying relation between them and the nephridia.
In Polynoe the nephridia are short tubes with a slightly folded funnel whose lumen is intercellular, and this intercellular lumen is characteristic of the Polychaetes as contrasted with leeches and Oligochaetes.
- The Oligochaeta agree with the leeches and differ from most Polychaeta in that they are hermaphrodite.
These worms lay cocoons like the Oligochaeta and leeches, and where they depart from the structure of the Oligochaeta agree with that of leeches.
The ganglia are crowded at the posterior end of the body as in leeches, and there is much tendency to the obliteration of the coelom as in that group. Pterodrilus and Cirrodrilus bear a few, or circles of, external processes which may be branchiae; Bdellodrilus and Astacobdella have none.
- The leeches are more particularly to be compared with the Oligochaeta, and the following definition embraces the main features in which they agree and disagree with that group. Setae are only present in the genus Acanthobdella.
On the other hand, it has been held that in these leeches there is no vascular system at all and that the entire system of spaces is coelom.
A stage exactly comparable to the stage in the leeches, where the ovary is surrounded by a closed sac, has been observed in Eudrilus.
We have here clearly a case of a true coelomoduct performing the function of an oviduct in both leeches and Eudrilidae.
The rivers and lakes yield enormous quantities of fish, and leeches also are plentiful.
Classes: Arachnida, Insecta (including Sub-Classes M y riapoda, Hexapoda), Crustacea (including Sub-Classes Entomostraca, Malacostraca), Epizoa (Epizootic Crustacea), Annellata (Chaeto p ods and Leeches), Cirripedia.
Leeches were his favourite instruments, and so much so that he is said to have used ioo,000 in his own hospital wards during one year.
Leger (32, 33) and others, that leeches are true alternate hosts for these forms, in which certain phases of the life-cycle are normally undergone.
Certainly some of the medical practices of the ancient world, such as bloodletting and the use of leeches, seem to us at least misguided and at worst, barbaric.