NATURAL HISTORY OF BRITISH FRESH-WATER LEECHES. 65
which are repeated at intervals—each repetition corresponding
to a somite. The number of rings to a somite is greatest in the
middle region and decreases towards the extremities, where a
somite may possess only one ring.
Distinct patterns are often to be recognised on the dorsal
surface, and where these are of use in the identification of species
attention will be called to them.
A well marked sucker, disc-like in form, is to be seen at the
posterior extremity. The mouth functions as an anterior
sucker, but it is not, as a general rule, disc-shaped.
The eyes appear as black spots and are placed on the posterior
surface over the mouth region. They vary in number from
one pair to five pairs, and their number and position are of great
value in the determination of species.
The alimentary canal consists of a straight tube extending
from the mouth to the anus, the latter being situated on the
dorsal surface, a little in front of the posterior sucker. The
alimentary canal is often provided with lobes which serve to
store the food until digestion is completed.
In certain species the mouth is provided with three jaws which
are armed with a large number of teeth. Others again do not
possess jaws, but have an extensile proboscis. A third series
have neither jaws nor proboscis. These characters serve to
divide the class in the following manner.
Sub-order I. RHYNCHOBDELLAE (Without jaws, but with
an exsertible proboscis.)
Family I. Ichthyobdellidae. (Body cylindrical in fresh-
water species. Anterior sucker discoidal. Eggs enclosed in
capsules attached to foreign bodies and left by the. parent
to hatch.)
Family 2. Glossosiphonidae. (Body ovate, flattened,
never cylindrical. Anterior sucker not conspicuous. Eggs
deposited in thin capsules either fixed to a foreign body or to
the body of the parent, but, until hatched, always brooded by
the parent, to whose body the young then attach themselves.)
Sub-order II. ARHYNCHOBDELLAE. (Without a proboscis.)
Family I. Gnathobdellidae. (With jaws.)
Family II. Herpobdellidae. (Without jaws.)
Space will not permit me to enter into a description of the
vascular, excretory and nervous systems. These are fully