CXll 
THE VOYAGE OF H.M.S. CHALLENGER. 
group, under the name Streptastrosa ; they also are still more generally characterised by 
an aster of some form, whether spiraster, or euaster, or spheraster {Calthropella). 
The Stellettidse are a compact and natural family, the megascleres are usually radiately 
arranged, and an ectosomal trisene, which may be an orthotrisene, plagiotrisene, or 
dichotrisene, is invariably present, the mesoderm of the choanosome is without exception 
sarcenchymatous, and the chamber-system aphodal ; these characters alone suffice to 
define it from the Sigmatophora and the Streptastrosa ; the family is therefore one of 
particular value in an inquiry into the value of the microsclere in classification. We notice 
then in the first place that a euaster is present in every species of the family, and while in 
many genera an additional microsclere is present, this is never a sigmaspire, nor a spiraster, 
but either a second form of euaster, or a diactinose aster (microrabd) or a sanidaster, {i.e., 
a chiaster with a rhabdal axis instead of a centrum), or an irregular amphiaster, which is 
similar to a sanidaster, but of uncertain origin. 
The sanidaster and irregular amphiaster are the only microscleres in the family 
which are not centrastrose, and the amphiaster is the only elongated aster occurring in 
other groups, but even it presents differences which distinguish it from the amphiaster 
of the Streptastrosa, and more nearly resembles a sanidaster. The microscleres of the 
Stellettidse thus adhere with marked persistency to the euastral type, yet not so closely 
as to render them infallible guides, for were a Thenea with amphiasters and euasters to 
acquire a sarcenchymatous mesoderm and aphodal chamber-system, one would feel bound 
to include it in the Stellettidse. The nearest approach to such a complement of spicules 
as that suggested occurs in Thenea delicata, which possesses amphiasters and plesi- 
asters. 
The Stellettidae, so far as we know at present, are the only family of the demus 
Euastrosa, but there are certain Monaxonid Sponges which may eventually have to be 
included; these have been placed in a family Epipolasidse, as an Appendix to the 
Euastrosa ; the character of the mesoderm and of the chamber-system is not known in 
those genera of the family which in their spiculation make the nearest approach to the 
Stellettidae, e.g., Asteropus [Stellettinopsis, Carter), which possesses oxyasters and 
sanidasters ; in the only example {Amphius huxleyi), in which the chamber-system is 
known it agrees with that of the Stellettidse, but this species possesses only one form of 
microsclere and that an amphiaster. The Epipolasidse are not only without trisenes, 
but the oxeas do not strictly adhere to a radial arrangement. There is another genus 
of the family which departs still further from the Stellettid type (Coppatias), and which 
I think should most probably be included with the Monaxonida, but as I have not had 
spirit specimens of these Sponges to examine I leave the question open. 
The next family is the Geodiidse, a very homogeneous group, characterised by the 
presence of the sterraster, but the constancy with which this spicule is present is of no 
service in the present inquiry, for were it absent the Sponge would become a Stellettid, 
