Mr. C. Collingsvood on Oceanic Forms of Hydrozoa. 309 



the body of the fish whilst swimming; and it is evident that, 

 m 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. — RemarJcs upon Oceanic Forms o/Hydrozoa observed at Sea. 

 By CUTHBERT COLLINGWOOD, M.A., F.L.S. &c. 



The 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 Physophoridse and Luccrnaridaj. 

 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- 

 tained 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 Medusid^e 

 (naked-eyed Medusae) 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 Medusae, as well as Beroes and similar 

 oceanic Actinozoa, also small gelatinous masses usually more or 

 less torn by contact with the net. The pelagic species of the 

 order Physophoridse, 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 PorpiUe, 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 i-ange very remarkable. 

 I have found them extending over 55° of latitude, and I have 

 no reason to believe this to be the limit. 



