cxlvi 
THE VOYAGE OF H.M.S. CHALLENGER. 
the generic distinctions of the group depend, these will be best understood by reference 
to the accompanying diagrams (Fig. XIV.). 
Skeleton. — The general arrangement of the megascleres is radial, but to this there are 
several exceptions, at least in the adult state, for it is probable that in the very young 
state all the Geodiid species are radiate. 
The most marked exceptions among adult Sponges are met with in Erylus, Caminus, 
and Pachymatisma ; in the last named the oxeas are scattered without any trace of order 
Fig. XIV.— Diagrams of the canal system as distinctive of the genera of Geodiidae. A, Pachymatisma ; B, Cydonium ; 
C, Oeodia ; D, Synops j E, Isops ; cr.ch., cribriporal chones ; u.p.ch., uniporal chones ; o., oscule ; (i) incurrent, 
(e) excurrent ; s, sphincter. 
through the Sponge, but the trisenes, which are as a rule confined to the neighbourhood 
of the cortex, are radially disposed ; in Erylus the arrangement is essentially the same, 
and it is very similar in Caminus, the chief difference lying in the more general 
aggregation of the oxeas into fibres, which are directed radially near the cortex, but 
wander without any definite direction elsewhere. In both Caminus and Pachymatisma 
the oxeas are frequently cemented together by spongin. 
