12 THREE CRUISES OF THE “BLAKE.” 
The sponges also seem specially to dwell upon the continen- 
tal slopes, and here it is that the kingdom of brightly colored 
sponges displays its splendor of yellow, orange, red, and brown. 
The sponge zone is comparatively narrow on the bank of Flo- 
rida, where perhaps it takes its greatest development in the 
districts explored by the “ Blake ;” it disappears at about one 
hundred and fifty fathoms, sometimes before, particularly where 
the bottom affords favorable conditions for the deposition of silt 
or ooze, which is destructive to the development of all except 
the siliceous sponges. 
The Lithistidze and Hexactinellidee do not occur in the littoral 
zone, while the other families, though often extending into deep 
water, also run into the littoral zone, but take their principal 
development between one hundred and two hundred fathoms. 
The dredgings of the “Blake” reached from shallow water, 
generally within the hundred-fathom line, to the abyssal depths 
of the same area. "These dredgings therefore give us terms of 
comparison for the inhabitants of all depths of the same region, 
many of which are missing from the collections of the other 
deep-sea explorations, as they ceased work when approaching 
the shore line. 
We are thus able to trace far more accurately than we could 
from other collections, not only the species which are merely 
littoral and have migrated into deeper water, often at a con- 
siderable distance from their original littoral habitat, but also 
those which after migration have become modified so as to form 
the characteristic faunal inhabitants of the continental and abys- 
sal regions, and those cosmopolitan species, assumed to be of 
arctic or antarctic origin, which have an immense geographical 
range over the whole bottom of the Atlantic and Pacific oceans. 
The last may be considered stragglers or colonies, which have 
found their way, towards both the littoral and abyssal regions, 
into faunal districts not strictly their own, according to the dis- 
tance of deep water from the shores, or the nature and direction 
of currents. We may thus get a most striking contrast be- 
tween the faunz of adjoining littoral, continental, and. abyssal 
regions. This is shown by paleontological evidence from dis- 
tricts corresponding to the shallower continental regions of our 
day. 
