CXXXVl 
THE VOYAGE OF H.M.S. CHALLENGER. 
Definition. — Euastrosa in which the megascleres are oxeas and tri^nes. The 
chamber-system is aphodal and the mesoderm of the choanosome sarcenchymatous. 
The ectosome may or may not form a cortex. 
There may be but one form of aster or there may be two or more forms present ; in 
the former case the additional aster is usually a euaster or a euaster reduced to a 
microrabd, but there are some few cases [Trihrachium, Tethyopsis, Disyringa) in which 
it is a sanidaster, and one in which it is an irregular form of amphiaster (Stryphnus), 
The arrangement of the megascleres is as a rule on the “ radiate type,” even when 
absent in large fully grown Sponges this arrangement usually occurs in the young 
Sponge ; but there are one or two cases in which there is an almost total absence of 
arrangement in all known stages, e.g., in Stryphnus, and the species Stelletta pathologica, 
0. Schmidt, of which the generic relations are obscure. 
The triaenes invariably present are usually ortho- or dicho-triaenes, the former, how- 
ever, sometimes replaced by plagiotriaenes ; anatriaenes may or may not be present in 
addition. In Stelletta pathologica remarkable centrotriaene forms are met with, and 
these attain their full development in the choanosome as well as in the ectosome, while 
in the rest of the family the triaenes are never fully grown till they reach the ectosome 
or its vicinity. 
In the following genera the ectosome does not form a cortex : — Myriastra, Anthastra, 
Ecionema; in the following the cortex is chiefly spicular ; — Aurora, Papyrula, Stryphnus, 
Algol ; in the remainder chiefly flbrous or flbro-spicular ; — Pilochrota, Astrella, 
Psammastra, Trihrachium, Ancorina, Disyringa, Tethyopsis. 
Classification of the Stellettidse. — The only attempt yet made to reduce the species 
of the overgrown genus Stelletta (for our Stellettidae is almost equivalent to Schmidt’s 
genus) to order is that of Carter, whose classification is as follows : — 
“ Group Stellettina. 
“ Subsection I. Psilodermata : — ‘ Thin-skinned Stellettae.’ 
“ a. Cortex thin or next to nothing, charged more or less with minute stellates 
only (Stellifera). 
“ h. Cortex similar, but charged with bacilliform bodies chiefly (Bacillifera). 
“ Subsection II. Pycnodermata : — ‘ Thick-skinned Stellettae.’ 
“ a. Cortex thick, charged with discoid bodies (Discifera). 
“ h. Cortex thick, charged with globostellates (Globostellata).” 
No illustrative species of Stellifera are given ; of Bacillifera, Ecionema acervus, 
Bwk., Ecionema densus, Ecionema ponderosus Stryphnus ponderosus), Bwk., 
and Stelletta helleri, 0. S., are quoted; of Discifera, Stelletta euastrum, 0. S., Stelletta 
discophora, 0. S., Stelletta mamillaris, 0. S.; and of Globostellata, two new species, 
Stelletta reticulata, Crtr., and Stelletta globostellata, Crtr. 
