HIRUDO. 3 
sucker in front is relieved, carried forward, and secured, when the pos- 
terior is relieved, carried forward, and secured in like manner, whereby 
the animal has taken its step. This process is repeated, while adhesion 
constantly prevails. The Leech also swims quickly by undulations in the 
water. It feeds on animal and vegetable substances ; and it multiplies 
by means of ova. 
§ 1. Hirvupo muricata—The Skate Leech.—Plate I. fig. 1. 
Length eight inches ; extreme diameter seven lines. Body round, 
tapering slightly from the middle towards the two extremities, each of 
which dilates as a large circular sucker. The whole body profusely 
tuberculated ; the tubercles rising about a line, disposed in regular circles, 
and susceptible of being flattened. A portion of the neck near the head 
differs somewhat from the rest, in forming a broad belt of lower tubercles. 
Neither eyes nor ocular specks are to be discovered. 
The body is annulated, as may be discovered from young specimens, 
for this becomes imperceptible with their growth. Between seventy and 
eighty distinct circles were enumerated in one extending about two 
inches in length, by a line in thickness, the tubercles seeming to be ar- 
ranged in about eight longitudinal rows. 
During quiescence the animal adheres firmly by the posterior sucker ; 
it forms in its extent a logarithmic curve, with the oral disc expanded 
or otherwise in the-centre. Now the disc appears as a hollow hemi- 
sphere, which unfolds as a flattened surface, to be applied to neighbouring 
objects when the animal commences its progression. While altogether 
disengaged, the lip of the hemisphere contracts so as to render it spheri- 
cal. When employed, it is unfolded flat ; the mouth appearing in the 
centre, environed by concentric rings of different lines. The exterior 
ring was in one specimen greenish, the next white, very narrow, within 
a broad grey ring. Eight low tubercular prominences are disposed on the 
margin of this oral sucker ; sometimes four, as were on the disc just 
described. There is a considerable contraction of the diameter of the 
