20 
Dr. Johnson's observations 
six to twelve. One of them deposited, from the 8th of 
October to the 29th of the same month, no less than twelve 
capsules, and taking an average number of the ova they con- 
tain, and supposing only one-third to be productive, we 
should have thirty-six young from this single leech. 
From the similarity of the Hirudo vulgaris , both in its struc- 
ture and general appearance, to the Hirudo medicinalis, I think, 
we have every reason for believing that the latter is also 
oviparous ; and that the ova have an enveloping membrane, 
which is formed and deposited, after the manner already 
mentioned. 
From what has been advanced, we learn, 
1. That the Hirudo vulgaris copulates in the same manner 
as the common snail. 
2. That it is oviparous, 
3. That the ova are imbedded in a gelatinous mass enve- 
loped by a strong semi-transparent fibrous membrane^ deno- 
minated the capsule, 
4. That the capsule contains from six to twelve ova, which 
are globular, and have a smooth surface. 
5. That these ova in the course of a fortnight, lose their 
globular shape, and show jagged edges : that in three weeks, 
they take the form of an oblong oval, when we discover 
animal existence : and in six weeks, the young make their 
escape. 
6 . That the number of the capsules each leech produces., 
(one every second or third day) varies from six to twelve. 
7. That the young are nearly colourless at the time of birth, 
and continue so for many months, increasing little as to size, 
