398 
THE VOYAGE OF H.M.S. CHALLENGER. 
continental depths, and descends a little more rapidly towards the shore than towards 
deep water ; from this and the similar behaviour of the first curve it would appear that 
Sponges generally, and the Monaxonids in particular, are more generally distributed in 
water of from 51 to 200 fathoms deep than in depths of from 0 to 50 fathoms, but on 
the other hand those localities in the shallower water where they do occur are com- 
paratively richer in species. 
The curve of Hexactinellid Sponges culminates on the ordinate III., from which we 
may infer that the characteristic depth for this group lies between 201 and 1000 fathoms, 
so that this is pre-eminently the deep-water group of Sponges ; it descends more rapidly 
towards deeper water than towards the depth of 51 to 200 fathoms, and it does not reach 
the first column at all ; all the same Hexactinellids do extend into shallow water, since 
0. Schmidt records the occurrence of Cystispongia superstes, off Yucatan, in 18 fathoms. 
This will prevent our placing too much confidence on the results obtained by a single 
expedition ; the dredgings will require to be enormously multiplied before we shall be in 
possession of sufficient statistics to enable us to frame completely satisfactory tables. 
Schulze questions the accuracy of Schmidt’s record in the case of Cystispongia superstes 
just cited, because the given depth appears to be exceptional, but this is scarcely 
sufficient reason for setting aside Schmidt’s direct statement ; that the fact is exceptional . 
gives it greater interest. On comparing the station curve with the species curve in the 
case of the Hexactinellida it will be observed that while both culminate on the ordinate 
III., the former diverges from the latter towards shallow water, but approaches it 
towards the abyssal depths, showing that stations are richer in species in the shallower 
than in the deeper water. 
The Tetractinellid curve comes next in order ; it culminates on the ordinate II., so 
that the Tetractinellida appear to flourish best in water of from 51 to 200 fathoms deep ; 
even here where they are most numerous they fall below the Hexactinellida in numbers 
and far below the Monaxonida. The slope of the curve shows that they faU off“ much 
more rapidly towards the deeper than towards the shallower water. The station curve 
is most remote from the species curve on the ordinate II., that on which the species 
curve culminates, and approaches it towards the ordinates for both deeper and shallower 
water. Hence so far as the Challenger observations are a guide it would appear that the 
localities which are richest in Tetractinellid species occur in water of between 51 and 200 
fathoms deep, but that on the other hand more numerous localities for Tetractinellida 
occur in water of from 0 to 50 and from 201 to 1000 fathoms deep. This result may not 
be true of Tetractinellids in all circumstances, but only of such as are obtainable by the 
dredge or trawl, which obviously cannot bring up to the surface species which incrust 
the under sides of reefs and rocks, a locality especially affected by many Sponges, 
including Tetractinellids. 
The curves for the Ceratosa and Calcarea may be passed without comment. 
