The Calcarea and Ceratosa are strictly shallow-water forms. It is a fact well worth consideration that the stations at which sponges have been found are situated, quite irrespective of depth, more or less in the neighbourhood of land. In the case of Calcarea and Ceratosa this is to be expected, seeing that shallow water is commonest near land, but it is surprising that it should be true also of the Hexactinellida and of the deep-water species of Tetractinellida and of Monaxonida. While the family groups are cosmopolitan, this is not true of genera and species. The distribution of genera and species makes it possible to define certain geographical provinces for sponges as for other animals. That this is so, is due to the existence of ocean tracts bare of islands; for ocean currents, can act as distributing agents with success only if they flow along a coast or across an ocean studded with islands. It is, of course, the larval forms which will be transported; whether they will ever develop to the adult condition depends on whether the current carrying them passes over a bottom suitable to their species before metamorphosis occurs and the young sponge sinks. If such a bottom is passed over, and if the depth is one which can be supported by the particular species in question, then a new station may thus be established for that species. The distance over which a larva may be carried depends on the speed of the current by which it is borne, and on the length of time occupied by its metamorphosis. Certain of the ocean currents accomplish 500 miles in six days; this gives some idea of the distance which may intervene between the birthplace and {241}the final station of a sponge; for six days is not an excessive interval to allow for the larval period of at any rate some species. DISTRIBUTION IN TIME.—All that space permits us to say on the palaeontology of sponges has been said under the headings of the respective classes. We can here merely refer to the chronological table shown in Fig. 123:[281]— [Illustration: FIG. 123.—Table to indicate distribution of Sponges in time.]