16 THREE CRUISES OF THE " BLAKE. 



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 Theel, 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 Psolidae, 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 palaeozoic, 



