In contrast to the previous class of sponges, the gelatinos sponges contain a jelly, as the name would suggest. This jelly fills the interstices of the body tissue, and the flow of water is thus restricted to a system of inhalant and exhalant canals.
These are sponges of the gelatinose type, and the spicules are of calcite. In addition the spicules have a triradiate form, or some modification of the three-rayed pattern.
Ascon sponges (Suborder Homocoela)
This suborder consists of calcareous sponges where the whole of the cloacal cavity is lined with collared cells. Examples are Leucosolenia and Clathrina.
More calcareous sponges, but in this suborder, the collared cells are restricted to distinct chambers. These chambers are either thimble-shaped, as they are in the glass sponges (Hexactinellida), or small and rounded with inhalant and exhalant canals. Typical genera of the suborder are Scypha (=Sycon) and Leuconia (= Leucandra).
These are gelatinose sponges in which the spicules have the form of a tetract or tetraxon, and are consequently four-rayed. Frequently, however, the spicules are modified to form a monaxon or one rayed spicule. In this order the size of the spicules varies enormously, and to facilitate identification they are divided into two main categories, large and small. The large spicules are known as mega scleres, and the small as microscleres.
In this suborder differentiation of the spicules into megascleres and microscleres is slight, and in some cases there is none at all. Typical genera of the suborder are Plakina and Oscarella.
Tetraxonid sponges with star-shaped microscleres based on a twisted form, which are consequently known as streptasters. A typical example of this kind of sponge is Thenea.
In this suborder the spicules are microscleres based on rays that emanate from a common centre, and are known as euasters. Examples are Stelletta, Cliona and the Loggerhead Sponge (Spheciospongia).
The microscleres in this suborder are based on C- or S-shapes, and the typical genera are Myxilla, Haliclona and Halichondria.
Horny sponges (Order Keratosa)
These are gelatinose sponges with a skeleton that consists entirely of horny fibres. The fibres are interlocking and consist of a silk-like protein material that is known as spongin. A typical example is Spongia, the bath sponge.