While we were doing the research for this page, we came across a much better on-line source of information than we could ever produce. Thus, we will keep this page short, and commend any readers to Guideline to the morphological species description for the Sponge Barcoding Database (SBD). That site also refers to Hooper (2000), which is an excellent source, but the web version of Hooper's work has no illustrations. The Guideline has illustrations but no written definitions. For the most part, you won't need written definitions, but they are useful for getting grounded in the system. Hence, we supply the following as a sort of introduction to the most basic terms. Another useful source for spicule terms is Andri et al. (2001). It is most easily used in HTML format at this link. A useful review of sponge spicules in general, but not so convenient for issues of terminology, is Uriz (2006).
Actually, there ought to be some way of discussing sponges without getting enmeshed in the terminology of spicules. We actually tried to do this, but ultimately failed. The subject can't be ignored, won't fit in a glossary entry, and is simply too boring to slip into some phylogenetic discussion. We will restrict the discussion to megascleres -- the large, structural spicules. Megasclere terms are built from (a) a numerical prefix, (b) a structural suffix, and sometimes (c) some sort of language-specific grunt at the end.
Prefixes: nothing peculiar here, except the unholy mixture of Latin and Greek roots:
Mono- = 1
Di- = 2
Tri- = 3
Tetr- = 4
Hex- = 6
Suffixes: this is where it gets trickier. Many sources and some textbooks confuse these three:
-actine = rays
-axon = axes
-radiate = rays in a single plane
Grunts: ignore these
-al (unnecessary sur-suffix for adjectives)
A "ray" or actine is supposed to refer to a growth zone. That is, a ray theoretically represents the position of one terminal sclerocyte spicule-forming cell). Unfortunately, that particular theory can be difficult to put into practice. Multiple sclerocytes often cooperate on a single growth zone, split up to form branches or forks, or just wander around doing maintenance.
A more practical (and biologically reasonable) way to rationalize the nomenclature is to imagine each spicule element as an axis emanating from the center of the spicule. If the axis grows in only one direction out from the center, it is monactine. If the axis grows also grows in the opposite direction, it is diactine. So far as we know, these are the only two possibilities for sponge spicules.
Perhaps there are sponges out there which build boomerang-shaped axes which curve sharply at the center, so that the two ends do not grow in opposite directions. Too bad. It will probably be described as diaxonic, not diactine. That is, it will be conceived as having two separate axes, rather than a single, curved axis with two growing ends.
Further, it makes no difference whether the axis curves, branches, or forks beyond the center. Such ornamentation, however Baroque, does not alter the count of either rays or axes. .
The system becomes a bit ambiguous in a number of cases. For example, some spicules look like a trident, with one long ray and three short, curved rays at one end. Is this a tetraxon, or perhaps an ornamented diactinic monaxon? The tendency is to treat such cases as tetraxons.
Finally, We have found one special case, with no obvious explanation. If we ever find the explanation, we will add it here. This case deals with an apparently simple tetraradiate diaxon: a '+' sign configuration. This is frequently classified as a triaxon. One possible reason is that spicules are small and fragile. Particularly with fossil materials, it can be impossible to tell whether the spicule is actually a tetraradiate diaxon, or the broken remains of a (much more common) hexactine triaxon (see image for an example). Unfortunately, the latter are diagnostic of Hexactinellida, so this practice can lead to problems.
 If we were able to write our own spicule nomenclature, we would include the terminology of Baroque ornamentation. This would allow for curved axes (appoggiaturi), terminal splitting (mordents), irregular turns (turns), and little sprays of spikelets (trills), all of which are as common in sponges as in Scarlatti.