160 DISTRIBUTION. 



or (lelermliie tlicm on the spot, our data for tlie Geographical Distribution of all but the Serlu- 

 larince, with their firm, chitinous, ])ersistent hydrotliecae, must continue very defective. 



It is, indeed, by no means improbable that the distributional relations of the Gymnohlastea 

 are in many respects very diff'erent from those of the CalyptoUastea, and that not only the species, 

 but a large number of the genera of the gymnoblastic hydroids are confined within limited areas 

 of distribution. Numerous liydroid medusae have been noticed by navigators in tropical and 

 other distant seas, and though these have not received the attention they deserve, there is reason 

 to believe that among them are many gcnerically different from any which frequent our own 

 latitudes. It is probable, as we have already said, that these medusae do not s])ontaneously 

 migrate far from the rooted trophosomes in which they originate ; and if so their presence would 

 indicate not only the existence of these trophosomes in the seas frequented by the medusae, but 

 their limitation within definite areas corresponding to those of the medusaB.' 



Now, with the exception of such few as may have originated directly from the egg, it is 

 almost certain that all these medusae are the planoblasts of either gymnoblastic or campanularian 

 trophosomes,— the two groups of which, as has been said, collections from otiier countries, have 

 hitherto afibrded us such few and imperfect examples. 



That a more complete knowledge of the gymnoblastic hydroids of other parts of the world 

 would result in the discovery of numerous generic forms, limited in their distribution to definite 

 areas, is rendered further probable by the fact that in almost the only region beyond tiie European 

 seas where they have been subjected to a scientific study, many generic forms, entirely diff'erent 

 from those of Europe, have been established. I refer to the researches of the American naturalists, 

 more especially to those of the two Agassizs, and of Clark, McCrady, and Stimpson, which have 

 resulted in the discovery of seven genera of gymnoblastic hydroid trophosomes inhabiting the 

 Atlantic shores of North America, and unknown upon the eastern side of the Atlantic. 



But few facts regarding the distribution of the Eleutherohlastea are known to us. The 

 freshwater genus Ili/dra, the only generic representative of this group, appears to be very widely 

 distributed, for it is not only spread througiiout nearly the whole extent of Europe, but it has 

 been recorded from North America and from Northern Afiica. The extra-European examples of 

 Hydra, however, require further examination before their diff"erence from European species or their 

 identity with them can be asserted with confidence. 



Though the species of cajyptoblastic hydroids have been proved to be on the whole allocated 

 to definite localities, a few would appear to be widely distributed. Serlnlaria polysonias, a very 

 abundant British species, is also one of the most common sertularians of tiie Mediterranean, where 

 I have found it ranging from Naples to Nice ; while, according to the observations and identi- 

 fications of Agassiz, Busk, Hincks, and others, it occurs upon the Atlantic shores of the United 

 States, and on the shores of Greenland, Norway, Madeira, South Africa, the Red Sea, and the Falk- 

 land Islands ; and I have also determined this species among liydroids collected in New Zealand. 

 Flumularia ohliqita, another British species, has, as already stated, been recorded by Hincks from 

 Australia. South Africa and Australia are given by Busk as localities for Serlidarla ojjercidata, 

 another abundant British species, while Ilincks appears to have also identified it among hydroids 

 from Patagonia, the Falkland Islands, the Auckland Islands, New Zealand, and Kerguellan's Land. 



^ The occurrence of IiyilroicI mcduscc at a great distance from tlie nearest laud is probably due 

 to the diiftiu'r action of winds and currents. 



