AIRUDO. 191 
The common leech (Aivudo medicinalis) is an elon- 
gated, slightly-flattened worm of freshwater habitat. It is 
found commonly in ponds and is usually of a dark greenish 
Hapits, nt with yel- 
low lines and pig pr7.—VewrraL Vinw oF THE 
spots. The ventral sur- precy. (Natural size.) (Ad nat.) 
face is darker, sometimes 
black. Like Avenicola, it 
is plano-symmetric. The 
mouth is at the anterior 
end in the Nephridiopore. 
centre of a 
sucker. It is 
triangular and armed with 
three chitinous teeth. At 
the hind end is a large Aperture? ~| 
posterior sucker and the 
anus is situated dorsal to 
It. 
As in Avenicola, we 
can see a great number 
of annuli or rings, of 
which about five are 
contained in each true 
segment, but there is a 
marked absence of gills 
and appendages. Airudo 
breathes by the skin and 
the suckers take the place 
of appendages in loco- 
motion. The segments 
can be made out by the 
presence on the first an- 
nulus in each of paired 
rows of sense-organs. In 
this way there can be 
found twenty-six  seg- 
ments, and development 
indicates that the posterior sucker is formed from seven 
segments fused together. Hence the body of the leech 
consists of thirty-three segments. 
Mouth. 
External 
Features. 
Aperture 6 —~ 
