362- The Structure and Habits of Physalia. 



THE STBUCTUBE AND HABITS OF PHYSALIA. 



BY G. C. WALLICH, M.D., E.L.S., F.G.S. 



The last number of the Intellectual Obseevee contained a 

 summary of the researches of the older writers on Physalia, 

 from the time of Alexander when it was first noticed by 

 Aristotle as occurring in the Mediterranean, to the year 1843, 

 in which we find it described by M. Lesson in his Histoire 

 Naturelle des Acal&phes. But, although the more obvious por- 

 tions of the Physalian structure had already been described 

 at the date of the last-named memoir, it remained for our dis- 

 tinguished countryman Professor Huxley, to advance our pre- 

 vious knowledge of the Oceanic Hydrozoa generally, and to 

 correct many of those erroneous views regarding their organiza- 

 tion and morphological relations which were due to the state- 

 ments of De Blainville, Lesson, and others. 



It is the object of the present paper to adduce what further 

 information has thus been rendered available, and, at the same 

 time, to put the reader in possession of some apparently novel 

 facts bearing on the history of Physalia which have fallen under 

 the writer's immediate observation. 



Professor Huxley's first memoir on Physalia was forwarded 

 by him in 1847, from the Australian Seas, to the Linnean 

 Society; the more detailed account appearing, however, in his 

 admirable work on The Oceanic Hydrozoa, constituting the 

 volume published by the Bay Society for the year 1858. 



We find it there stated that " the body of every hydrozoon 

 is essentially a sac, composed of two membranes/'' an external 

 and an internal, which have respectively been called the " Ecto- 

 derm" and "Endoderm."* This sac contains the nutritive fluid 

 which performs the functions of the blood in the higher animals, 

 and is circulated by means of cilia which generally invest the 

 inner as well as the outer membrane and, aided by the mus- 

 cular contractility of the body, constitute the only circulatory 

 and respiratory mechanism in the organisms under notice. 

 The two membranes may readily be traced in every part of 

 the structure. In the large bladder (which, by the way, closely 

 resembles the swimming bladder of a medium sized haddock in 

 dimension and general outline) they form an outer sac, or 

 " pneumatophore" as it is technically termed, within which 

 the true air-chamber, or " pneumatocyst," is enclosed; the 

 latter being in reality a secondary introverted sac, having the 

 same structure in its walls, and communicating with the outer 

 world by a minute contractile orifice at the point at which the 



* Hj Professor Allman. 



