SALT-WATER LEECHES. 275 
able that these leeches should be capable of taking in so much 
of that fluid, a very little of which suffices the superior animals 
for their sustenance.” It is still an unsolved question why the 
Hirudinide require more nourishment than creatures far above 
them in the scale of life. We can understand why the silkworm 
should eat a weight of mulberry-leaves heavier than its own body, 
at a single meal, because the creature is so rapidly growing and 
storing that silk which is to compose its winding-sheet. But the 
blood of a man or of a fish is the very essence of nutriment, and 
yet the leech will imbibe a large quantity. This cannot be 
expended on its growth, for it grows but slowly. What, then, 
can be the reason for its gluttony ? 
The medical leeches can distend themselves with seven and a 
half times their weight of human blood ; but the sea-leeches can 
only take in twice their own weight. Why is this? It may partly 
be accounted for by the difference of their construction. The 
thicker skin of the sea-leech prevents it from expanding as much 
as the fresh-water leech. This latter animal rejoices in eleven pairs 
of stomachs, but the other has only one; and again, the blood of 
a fish is by no means so nourishing as human blood, therefore 
the sea-leech ought to take a larger quantity to equal the voracity 
of its marsh representative ; and yet we find the reverse is the case. 
These questions, and many like them, are, as Pliny says, “impene- 
trable to human reason, and lie hid in the majesty of Nature.” 
The quantity of blood taken from the fish by its parasite is never 
sufficient to weaken it, but the only effect it seems to have is that 
the appetite of the fish increases. Indeed, this gentle ‘“ blood- 
” at times seems to improve the condition of the fish, as if 
letting 
the parasite were ordained to remove a superfluity. It has been 
said that parasites seldom attack an organ, but feast upon the 
product of that organ; and thus, although they gain their own 
subsistence from the animals they live upon, yet it is not to their 
vital injury. There are instances to the contrary, yet happily they 
are but few. If the parasite always were the cause of the death of 
the animal it inhabited, parasitism would soon be at an end, and 
the harmony of Nature disturbed. 
The Addiones reproduce their species by egg capsules, which 
S 2 
