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 Lithistidse and Hexactinellidae 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 faunae of adjoining littoral, continental, and abyssal 

 regions. This is shown by palaeontological evidence from dis- 

 tricts corresponding to the shallower continental regions of our 

 day. 



