Earthworms, on the other hand, have not such easy means of travelling from country to country; the assistance which the cocoons in all probability give to the smaller aquatic Oligochaeta cannot be held to be of much importance in facilitating the migrations of the earthworms. In the first place, the animals themselves are of greater bulk, and their cocoons are naturally larger, and thus less easy of transportation. Secondly, they are deposited as a rule upon dry land, where the chances of their sticking to the feet of birds would be less; and thirdly, they are often deposited deep in the ground, which is a further bar to their being taken up. Another possible method by which earthworms could cross the sea is by the help of floating tree-trunks; it is, however, the case with many species that they are fatally injured by the contact of salt water. There are, it is true, a few species, such as _Pontodrilus_ of the Mediterranean coast, which habitually live within reach of the waves; but with the majority any such passage across the sea seems to be impossible.[427] On the other hand, rivers and lakes are not a barrier to the dispersal of the group. There are a few species, such as _Allurus tetragonurus_, {371}which live indifferently on land and in fresh water; and even some habitually terrestrial species can be kept in water for many weeks with impunity. A desert, on the other hand, is a complete barrier; the animals are absolutely dependent upon moisture, and though in dry weather the worms of tropical countries bury themselves deep in the soil, and even make temporary cysts by the aid of their mucous secretions, this would be of no avail except in countries where there were at least occasional spells of wet weather.