Among the coral reefs of the Indian and Pacific oceans, and also off the southern coast of Africa there are found spiders of the genus _Desis_ which spend almost all their time under the surface of the sea, from which they only emerge at low tide. They construct very closely woven tents, impermeable to sea-water, which imprison air at low tide, generally choosing for the purpose some cavity which has been excavated by one of the burrowing molluscs. Beyond this we really know very little about them, and there is much difference of opinion as to the mode in which they obtain their food. Some writers state that they only leave their shelters at low tide to chase small crustaceans, and that when placed in vessels containing sea water they are quite helpless and soon drown. On the other hand one observer found that a species of _Desis_ was quite at home in a sea-water tank, in which it swam freely and even attacked and fed upon a small fish. Possibly different species of the genus behave in different ways, some being more truly aquatic than others, though it is certain that the troubled waters of a coral sea are not a very promising field for sub-aqueous operations. We know a great deal more of the mode of life of those Agelenids which have taken to living in fresh water. Indeed the subject of the water-spider, _Argyroneta aquatica_, is so hackneyed that in dealing with it we shall probably be telling the reader much of what he knows already, but that possibility must be risked. There is, then, in many of our lakes, ponds and slow-flowing rivers with a weedy bed, a spider which has entirely taken to a water life, and for which it is useless to search on land. It is a docile captive, and consequently a favourite subject for transference to an aquarium, where its habits can be observed at leisure. Its first care is to construct beneath the water a small dome-shaped web, open below, and it generally selects the under surface of the leaf of a water weed for the purpose of anchorage, though a ready-made shelter is often furnished by the empty shell of some fresh-water mollusc. Its next proceeding is to fill this retreat with air in a very ingenious manner.