This first category of parasites includes all those which are not enclosed, and which live at the expense of others, without losing the attributes and advantages of a wandering life; they are as free as the vulture or the falcon which pursues its prey. We shall not, however, include among them the parasitical kite of Daudin, which tears from the hands of the traveller a piece of the flesh which he is preparing in the open air, nor the small Egyptian plover, which keeps the teeth of the crocodile clean. The former is a pirate, a highway robber; the plover, on the contrary, is a kind neighbour, an attendant who performs valuable services. We are more correct in considering as parasites the Vampires (_Phyllostoma_), those audacious bats of South America, which settle on the sleeping traveller or his beasts, and suck their blood by means of the sharp papillæ of their tongue. These animals are winged leeches which bleed their victim and pass on. We place among free parasites the greater part of leeches, some insects, and a certain number of arachnida, crustaceans, and infusoria. As we have mentioned free messmates, so we have free parasites, 108 which take advantage of their host, but with prudence and economy; they ask from him nothing but his blood, and sometimes render him important services. Many of these animals, both messmates and parasites, have at present been only provisionally classified, and cannot be definitely arranged till more observations have been made. It is not always so easy as it may be thought to determine exactly the relations which certain animals have with each other. We must pry very narrowly before we can ascertain the motives which act on this inferior order of beings. It is among free parasites that we find those organisms which are generally called vermin, and which seem the more capable of injuring their neighbours since they can the more easily escape detection. These creatures, though they are called vermin, excite no more repugnance in the mind of the naturalist than the other works of creation; and St. Augustine did not exclude them from his thoughts when he exclaimed, "_Magnus in magnis, maximus in minimis_." Leeches drink the blood of their victim, and when they are gorged to the very lips, they fall off, taking a siesta for weeks or months. Thus enjoying a repast at very long intervals, it is useless for them to continue longer at table; and this is therefore another reason that they should usually preserve their organs of locomotion, that they may use them after their long period of digestion.