i2 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
long distances by currents. It is not an uncommon thing to find the 
young of Arbacia, of Strongylocentrotus, and of Echinarachnius on our 
coasts floating about, and they are not unfrequently caught in the sur- 
face townet. The same holds good for many species of Starfishes and of 
Ophiurans, as well as of Holothurians. In Florida I have canght in the 
same way the young of Cidaris and Hipponoé, and of Toxopneustes, and 
of many other species of Starfishes and Ophiurans. These young Echino- 
derms all float, and may be carried very long distances during the period 
in which they still have the huge embryonic tentacles characteristic of 
their younger stages, when the ambulacral feet are entirely out of pro- 
portion in size to the rest of the test, and the young thus possess a great 
floating capacity when their suckers are expanded. 
They retain these suckers for a considerable period of time, during 
which they can be transported very great distances. There is no other 
explanation for the identity of the littoral marine fauna of the Bermudas 
than that the young and embryos of the Echinoderms and Polyps of the 
West Indies have been carried northward fully six hundred miles by 
the Gulf Stream at arate of from one to three miles a day, and have 
finally settled in the Bermudas. 
We can well imagine an equatorial current taking during Miocene and 
Eocene periods the young of the Echini flourishing in the Crag and in the 
Mediterranean, and in the southern extension of that fauna perhaps only 
from the Cape Verd Islands, and bringing them to the shores of North- 
ern South America or into the Caribbean Sea. That stretch is but 
little longer than the stretch which we know is annually traversed by 
Acalephs, Pteropods, Fishes, and Annelids, along the course of the Gulf 
Stream from the Straits of Florida to Narragansett Bay, and to the 
southern shores of Cape Cod and the adjacent islands. 
The existence of a continent or of intervening islands does not seem 
to me necessary to explain the similarity of the Echinid fauna of former 
times on both sides of the Atlantic or Pacific. The causes now at work 
appear to me sufficient to explain their relationship, when we take into 
account what is known of the efficient transporting agency of equatorial 
or other oceanic streams for the Pluteus or the young stages of Echini 
during a considerable period of their post-embryonic life. 
We should also remember that, even with our imperfect knowledge of 
the bathymetrical range of Echini, the range in depths of many genera 
is known to be very great, as will be seen from an examination of the 
lists given in the ‘‘ Challenger” Reports and from the depths obtained 
by this Expedition. Among these I may mention those having a great 
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