We have seen that the class Crustacea (crabs, shrimps, etc.) is the great division of the Arthropoda entirely adapted to an aquatic life, breathing, by means of gills, the air which is dissolved in the water. Insects and spiders are air-breathing, and properly belong to the land; yet there are many insects which pass their early stages—often the greater portion of their life—in the water, and some which are very fairly at home there when adult. Such insects often have gills when young, and are therefore at that period true water animals, like the Crustacea. The Arachnida—that division of the Arthropoda to which the spiders belong—include a few groups which permanently inhabit the sea, and could not live on land. There are even some weird creatures called Sea-spiders (Pycnogonids), but these do not concern us, for they are very far removed from the true spiders which are the subject of our investigations. Now the true spiders are always air-breathing, and if they venture into the water at all they must frequently come up to the surface to breathe, or else they must store up a reservoir of air beneath the surface of the water if they are to avoid death by drowning. Nevertheless some of them have been hardy enough to encroach on the domain of the Crustacea. Not a few are able to run freely on the surface of the water and even to dive occasionally for the purpose of seizing one of its denizens, but the number of those which have succeeded in really adapting themselves to aquatic life is very limited, and is, as far as we know, restricted to two small groups, both of them members of the Agelenidae.