THREE CRUISES OF THE “BLAKE.” 
16 
About as many species are identical with those of the An- 
tarctic Sea and the southern extremity of South America. The 
species which attain the greatest depth are usually those which 
have a very wide geographical distribution, generally with an 
arctic or antarctic connection, or they may be species dating 
back to the tertiary and cretaceous periods. 
The similarity of the holothurians of the arctic and antarctic 
regions has been recognized by Théel, but no species are com- 
mon to the two seas; it is therefore not probable that there is 
any interchange between the fauna of those distant regions, 
although in former ages such a connection may have existed. 
from the wider geographical -range of their progenitors; it is 
interesting to note in this respect, that in the Psolid», which 
find their way into very deep water, and have representatives in 
the tropic, temperate, and arctic zones, it is often most difficult 
to draw the specific limits. Still there are slight differences, in- 
dications of the changed physical conditions and various modes 
of life, which have caused the species to disappear from the in- 
termediate localities. The same resemblance is noticed among 
the sea-urchins, the starfishes, and the ophiurans. 
One of the most remarkable instances of the geographical 
extension of some genera is that of certain species of the family 
Lithodina. Professor Sidney I. Smith says: “These crustacea 
have been known as inhabitants only of the arctic and antarctic 
regions, living in the littoral zone; but now they have been 
found under the tropics, the only difference being that in this 
latter locality they have contrived to find congenial conditions 
of existence by abandoning their shallow-water life and betaking 
themselves to the cool depths of over 1,000 metres. This fact 
is not without its interest, showing us how some forms can 
spread from the frozen seas of the north to the seas of the 
tropics, and so from one pole to the other; altering their 
conditions of life as necessity demands, and resuming their old 
habits when the opportunity to do so again occurs.” 
Several species of sea-urchins are cosmopolitan; a number 
thus far seem peculiar to the Atlantic or to the Pacific, and 
these types all have a great bathymetrical distribution, or are 
representatives of fossil families that go back to the palæozoic, 
