Phoriospongia, as far as its main skeleton is concerned, is a typical Ceratose sponge, with fibres of the areniferous type, but it possesses sigmata free in the flesh.
The sub-class is confined to shallow water, no horny sponge having been dredged from depths greater than 410 fathoms.[[253]] The greatest number occur at depths between 10 and 26 fathoms.
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 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.
| 1. | (a) | Skeleton calcareous | [2] |
| (b) | Skeleton siliceous | [6] | |
| (c) | Skeleton horny, or without free spicules | [53] | |
| (d) | Skeleton absent | [55] | |
| 2. | (a) | Gastral layer continuous | [3] |
| (b) | Gastral layer discontinuous, confined to chambers | [4] | |
| 3. | (a) | Equiangular triradiate systems present | Clathrina |
| (b) | Triradiate systems all alate | Leucosolenia | |
| 4. | (a) | Chambers tubular, radially arranged | [5] |
| (b) | Chambers spherical, irregularly scattered | Leucandra | |
| 5. | (a) | Tufts of oxeate spicules at the ends of the chambers | Sycon |
| (b) | Oxeate spicules lying longitudinally in the cortex | Ute | |
| 6. | (a) | All the spicules hexradiate or spicules easily derived from hexradiate type | [7] |
| (b) | Some of the spicules calthrops or triaenes | [10] | |
| (c) | Megascleres uniaxial | [15] | |
| 7. | (a) | Amphidiscs present | [8] |
| (b) | Amphidiscs absent | [9] | |
| 8. | (a) | Rooting spicules a well-defined wisp; four apertures lead into the gastric cavity | Hyalonema thomsoni |
| (b) | Rooting tuft diffuse; sponge oval; osculum single | Pheronema carpenteri | |
| 9. | (a) | Sponge tubular, dermal and gastral pinuli absent | Euplectella suberea |
| (b) | Sponge a widely open cup; dermal and gastral pinuli present | Asconema setubalense | |
| 10. | (a) | Tetractine spicule, a calthrop or triaene with short rhabdome; microsclere a spined microxea | Dercitus bucklandi |
| (b) | Triaenes with fully developed rhabdome | [11] |
