HIRUDO. 13 
o 
§ 5. UDONELLA cALIGORUM.—Vol. I. Plate LXVI. fig. 11. 
Among those ordinances for which human reason cannot account, 
is that paramount decree of Nature prompting living animals to destroy 
each other. 
Amidst such wonderful means to kindle the spark of life ; amidst 
the complex machinery to sustain it ; the care and precautions for its 
transmission, how can it be credited that there are countervailing 
agencies purposely devised for destruction ? 
Yet certain it is, that one-half of the animated world will readily 
devour the other; nay that one more powerful being will remain the 
only survivor among all its fellows. 
Neither numbers nor dimensions distinguish the assailants ; some 
are mere atoms; others bear along the most gigantic proportions. 
Myriads of diminutive creatures wage a deadly war against a single 
victim ; or myriads of victims fall an instantaneous prey to a single de- 
stroyer. The end is the same, the devoted must perish. 
But the modes of destruction are different. Sometimes the flesh is 
consumed ; sometimes the blood is exhausted. 
In all this the final object of Nature can be scarcely mistaken, as, 
unless for the purpose of destruction, the victim would not have been 
provided. 
In as far as hitherto ascertained, all the species comprehended by the 
present genus fall within the class of destroyers. If doubts subsist of 
the fact, it is perhaps from the defects of history. Thus their sustenance 
is soft and succulent. 
Among the crustaceous animals, or those with shelly integuments, 
recently described, we have spoken of a parasite, the Caligus, which infests 
various kinds of flattened fishes. This singular parasite is itself infested 
by another, of altogether a different nature, and of infinitely smaller di- 
mensions. 
My attention was directed to it between twenty and thirty years 
ago, when first observing the Caligus. 
