In the majority of the Ceratosa the skeletal fibres are homogeneous, formed of concentric lamellae of spongin, deposited by a sheath of spongoblasts around a filiform axis. In others, however, the axis attains a considerable diameter, so as to form a kind of pith to the fibre, which is then distinguished as heterogeneous. In one or two cases some of the spongoblasts of a heterogeneous fibre are included in the fibre between the spongin lamellae. _Ianthella_ is the best-known example in which this occurs. Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished, as their names express, by the nature of the skeleton—net-like, with many anastomoses, in the one; tree-like, without anastomoses between its branches, in the other. The Dictyoceratina comprise by far the larger number of Ceratosa. They fall into two main families, the Spongidae and Spongelidae, both represented in British waters. The Spongidae {221}are characterised by a granular ground substance and aphodal chamber system; the Spongelidae by a clear ground substance and sac-like eurypylous chambers. The bath sponge, _Euspongia officinalis_, belongs to the Spongidae. The finest varieties come from the Adriatic, the coarser ones from the Dalmatian and North African coasts of the Mediterranean, from the Grecian Archipelago, from the West Indies, and from Australian seas. The softer species of the genus _Hippospongia_ also form a source of somewhat inferior bath sponges. Among Dendroceratina, _Darwinella_ is unique and tempts to speculation, in that it possesses isolated spongin elements, resembling in their forms triaxon spicules. KEY TO BRITISH GENERA OF SPONGES. {Skeleton calcareous 2 1. {Skeleton siliceous 6 {Skeleton horny, or without free spicules 53 {Skeleton absent 55 2. {Gastral layer continuous 3 {Gastral layer discontinuous, confined to chambers 4 3. {Equiangular triradiate systems present _Clathrina_ {Triradiate systems all alate _Leucosolenia_ 4. {Chambers tubular, radially arranged 5 {Chambers spherical, irregularly scattered _Leucandra_ 5. {Tufts of oxeate spicules at the ends of the chambers _Sycon_ {Oxeate spicules lying longitudinally in the cortex _Ute_ {All the spicules hexradiate or spicules easily derived from 6. { hexradiate type 7 {Some of the spicules calthrops or triaenes 10 {Megascleres uniaxial 15