PARASITES FREE DURING THEIR WHOLE LIFE.
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, 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.
Like the annelids, they do not change their form, and as they are only attached to their host for a short time, naturalists have not thought fit to place them among parasitical worms, or Helmintha. However, if we pass
from the higher kind of leeches to those which live at the expense of fishes, of crustaceans, and especially of molluscs, we see that the desire of possessing a lodging is developed by insensible degrees, and that the lower kinds, are by their form, their organization, and their mode of life, as dependant as the greater part of the helmintha. Thus we see Hirudinidæ on the Mya, an acephalous mollusc, incapable of quitting their place, firmly fixed on the walls of the stomach of their host, and living quietly at his expense. They are called Malacobdellæ, and they have been so ill-treated by Nature, that it is necessary to submit them to minute investigation in order to determine their parentage.
The most well-known leeches are those which attack man and the other mammalia, but some are also found on other vertebrate animals, especially on fishes. Their organization is always proportioned to that of the host which they frequent; thus, the simpler their host, the lower is their organization. The mollusc harbours hirudinidæ much lower in the scale than those which are found in fishes, and especially in mammals.
Vampires make use of the papillæ of the tongue, and also of their teeth, which act as so many lancets; leeches apply their toothed lip, saw asunder the epidermis, and with the mouth applied to a network of capillary vessels, suck till they fall off, intoxicated with blood.