20 bulletin: museum of comparative zoology. 



Haeckel, 1879, describes 30 species from the Canary Islands, 10 from 

 the coast of Africa and Cape Verde Islands, and 10 from the 

 "Tropical Atlantic." Maas, 1893, in his account of the Hydro- 

 medusa3 of the Plankton Expedition, enumerates about 21 additional 

 species; and 57 others have been made known by L. Agassiz, Brooks, 

 Fewkes, and Mayer from the Bahamas, Florida Reefs, and Tortugas. 



The Hydromedusan fauna of the Tortugas is so closely related to that 

 of the Florida Reefs and the Bahama Islands, that they may be said to 

 be practically identical; and we will therefore speak of it hereafter as 

 the " Bahama-Tortugas " fauna. 



When we come to compare the Hydromedusan fauna of the Bahama- 

 Tortugas with that of the remaining portion of the " warm zone," ex- 

 clusive of the West Indies, we are met with the remarkable fact that 

 only 7 species are known to be common to both the Bahama-Tortugas 

 region and the great remaining region of the " warm zone." Thus only 

 5 HydromedusiB have been found in both the Canary Islands and 

 Bahama-Tortugas region. These are vEginella dissonema, Aglaura 

 hemistoma, Aglaura hemistoma var. Nausicaa, Staurodiscus tetrastaurus, 

 and Laodicea ulothrix. Two other Hydromedusse, Glossocodon tenui- 

 rostris and Liriope scutigera, are found in the midst of the ocean 

 between the Canary Islands and the West Indies. It will be noticed 

 that 5 out of these 7 forms that are common to both the eastern 

 and western halves of the " warm zone " are Trachylina, or forms 

 that develop through a free-swimming planula and pelagic actinula 

 stage. The two others, Laodicea ulothrix and Staurodiscus tetrastaurus, 

 belong to the Leptolinidse and probably develop through a sessile 

 hydroid stage with alternation of generations. In 1893 it was shown 

 by Maas in " Die Craspedoten Medusen der Plankton-Expedition," 

 and in Natural Science, Vol. IT. pp. 92-99, that the great majority 

 of the Hydromedusse found in the midst of the Atlantic, far from 

 land, belong to the Trachylina, and the few Leptolina discovered always 

 show a relation to some neighboring coast. As is well known, it 

 was the avowed object of Hensen's Plankton Expedition of 1889 to 

 study the organic life of the high seas as free from the influence of 

 coasts as possible. This expedition entered the region that we have 

 designated the " warm zone " on August 20, and left it on October 20, 

 1889. During these two months the expedition remained for by far the 

 greater part of the time upon the higli seas, approaching land only at 

 the Cape Verde Islands, Ascension, Fernando Noronha, and the month 

 of the Amazon. As has been shown by Maas, 1893, the Hydromedusas 



