Mr. C. Collingwood on Oceanic Forms of Hydrozoa. 309 
the body of the fish whilst swimming; and it is evident that, 
in fishes moving during a great part of their life over swampy 
ground, or through more or less consistent mud, this function 
of the ventral fins ceases, and that nature can readily dispense 
with these organs altogether. 
XL.—Remarks upon Oceanic Forms of Uydrozoa observed at Sea. 
| By Curusert Cotiinewoop, M.A., F.L.S. &c. 
Tue following observations were made during a prolonged sea- 
voyage, extending over a year and a half, and embracing the 
Indian Ocean north and south of the Line, the China Seas, and 
the North and South Atlantic Oceans. During this voyage, of 
course, many interesting observations were made relating to 
other animals; but the present paper will be confined to the 
oceanic Hydrozoa of the orders Physophoride and Lucernaride. 
The appearance of these animals is by no means a constant 
occurrence even in calm seas, and seems in fact to be somewhat 
capricious and regulated by conditions which are not well ascer- 
tamed and would require the collation of a lengthened series of 
observations for their determination. On some days floating 
Hydrozoa occur in vast numbers; but when they do so, they 
usually appear to be all of the same species ; nor are such days of 
frequent occurrence. The small gymnophthalmatous Medusidz 
(naked-eyed Meduse) are so transparent that it is impossible 
to see them from the ship, although they may be tolerably 
abundant ; and it is only when they are captured in the towing- 
net that their presence is detected ; and that must be, of course, 
in calm weather, when the ship is not sailing too fast. But the 
towing-net is seldom put down without securing various forms 
of such transparent Medusz, as well as Beroés and similar 
oceanic Actinozoa, also small gelatinous masses usually more or 
less torn by contact with the net. The pelagic species of the 
order Physophoride, such as Velella, Physalia, and Porpita, 
occur perhaps more frequently than any others, and usually in 
company with one another, the two former especially seldom 
occurring one without the other, and having the appearance 
from a distance of large bubbles of water drifting by, though 
their persistence at once attracts the attention of the observant. 
' Next to them, perhaps, are Porpite, looking like beautiful blue 
gun-wads, with delicate radiating markings, and surrounded 
with a fringe of deep-blue tentacles. The number of these 
Hydrozoa must be enormous, and their range very remarkable. 
I have found them extending over 55° of latitude, and I have 
no reason to believe this to be the limit. 
