Page:Popular Science Monthly Volume 8.djvu/690

670  above the sea. If this be true, not only is the proposed inland sea an impossibility, but we must also relegate to the domain of fable the accounts of the great lake of Tritonis, or assign it to another locality.



HE fight for a foothold in the animal world brings the combatants into many strange relations, few of which are more curious and interesting than those existing between the creatures popularly known as parasites and the animals which furnish them support. In these relations all grades of pauperism and criminality are represented. There is the miserable wretch that lives entirely at the expense of others, finding it easier to die than to help himself; the poor weakling, willing enough to do what he can, but sure to starve to death if left wholly unassisted; the petty thief that sneaks into his neighbor's premises and steals a portion of his store; and the audacious robber that boldly appropriates another's substance, and not unfrequently adds murder to his list of crimes. In his entertaining and instructive work on "Animal Parasites and Messmates," from which this article and its illustrations are mainly taken. Van Beneden makes these different degrees of dependence the basis of a rough but convenient classification, by which he separates, what have hitherto been known as parasites, into three groups, named respectively messmates, mutualists, and parasites.

The messmate is one that takes his place at his neighbor's table to partake with him of the product of the day's toil. He does not live directly at the expense of his host, but, abiding with him, obtains thereby better opportunities for securing a supply of food. This mode of getting a living is very common, and a curious thing about it is that animals comparatively high in the scale of organization do not scruple to quarter themselves upon others of much inferior grade. The fish known to naturalists as fireasfer lives in this relation. He takes up his lodgings in the digestive tube of a holothurian, and, regardless of the rules of hospitality, appropriates a portion of all the food that enters. He thus manages to get himself served by another better provided than he is with the means of fishing. Dr. Greef found at Madeira a holothurian over a foot long, in which one of these fishes was enjoying a peaceful and vigorous existence. Other fishes besides the fireasfer have been found in similar quarters; indeed, the situation appears a very favorable one for this mode of life, since not only fishes but crustaceans here take up their abode, sometimes in considerable numbers. Prof. Semper has seen holothuriæ in the 